View source for WikiJournal Preprints/Planet Nine

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'''Planet Nine''' is a [[w:List of hypothetical Solar System objects|hypothetical]] [[w:planet|planet]] in the [[w:outer region of the Solar System|outer region of the Solar System]].<ref name=TAJ201601 /><ref name=P9H2019/> Its [[w:gravitational|gravitational]] effects could explain the unusual clustering of [[w:orbit|orbit]]s for a group of [[w:extreme trans-Neptunian object|extreme trans-Neptunian object]]s (eTNOs), bodies beyond [[w:Neptune|Neptune]] that orbit the Sun at distances averaging more than 250 times that of the Earth. These eTNOs tend to make their closest approaches to the [[w:Sun|Sun]] in one sector, and their orbits are similarly tilted. These improbable alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known [[w:Solar System|Solar System]] objects.<ref name=TAJ201601 /><ref name=Sheppard2014/><ref name=fatty>{{cite magazine
 |last         = Burdick
 |first        = Alan
 |date         = 20 January 2016
 |title        = Discovering Planet Nine
 |url          = http://www.newyorker.com/tech/elements/discovering-planet-nine
 |magazine     = [[The New Yorker]]
 |accessdate   = 20 January 2016
 |archive-url  = https://web.archive.org/web/20160121034701/http://www.newyorker.com/tech/elements/discovering-planet-nine
 |archive-date = 21 January 2016
  |df           = dmy-all
}}</ref>

This undiscovered [[w:super-Earth|super-Earth]]-sized planet would have a predicted mass of five to ten times that of the [[w:Earth|Earth]], and an [[w:elongated orbit|elongated orbit]] 400 to 800 times as far from the Sun as the Earth. [[w:Konstantin Batygin|Konstantin Batygin]] and [[w:Michael E. Brown|Michael E. Brown]] suggest that Planet Nine could be the [[w:Planetary core|core]] of a [[w:giant planet|giant planet]] that was ejected from its original orbit by [[w:Jupiter|Jupiter]] during the [[w:Nebular hypothesis|genesis]] of the Solar System. Others propose that the planet was captured from another [[w:star|star]],<ref name="Mustill_etal_2016" /> was once a [[w:rogue planet|rogue planet]], or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.<ref name=TAJ201601 />

{{As of|September 2019}}, no observation of Planet Nine had been announced.<ref name="Meisner_etal_2017b">{{cite journal |last1=Meisner |first1=A.M. |last2=Bromley |first2=B.C. |last3=Kenyon |first3=S.J. |last4=Anderson |first4=T.E. |year=2017 |title=A 3π Search for Planet Nine at 3.4μm with WISE and NEOWISE |journal=[[The Astronomical Journal]] |volume=155 |issue=4 |page=166 |arxiv=1712.04950 |bibcode=2018AJ....155..166M |doi=10.3847/1538-3881/aaae70}}</ref><ref name="Perdelwitz_etal_2018"> {{cite journal |last1=Perdelwitz |first1=V.M. |last2=Völschow |first2=M.V. |last3=Müller |first3=H.M. |year=2018 |title=A New Approach to Distant Solar System Object Detection in Large Survey Data Sets |journal=[[Astronomy & Astrophysics]] |volume=615 |issue=159 |pages=A159 |arxiv=1805.01203 |bibcode=2018A&A...615A.159P |doi=10.1051/0004-6361/201732254}}</ref> While sky surveys such as [[w:Wide-field Infrared Survey Explorer|Wide-field Infrared Survey Explorer]] (WISE) and [[w:Pan-STARRS|Pan-STARRS]] did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System.<ref name="bnb20160617" /><ref>{{cite journal |last=Luhman |first=Kevin L. |date=2014 |title=A Search for a Distant Companion to the Sun with the Wide-Field Infrared Survey Explorer |journal=[[The Astrophysical Journal]] |volume=781 |issue=4 |page=4 |bibcode=2014ApJ...781....4L |doi=10.1088/0004-637X/781/1/4}}</ref> The ability of these past sky surveys to detect Planet Nine were dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using [[w:NEOWISE|NEOWISE]] and the 8-meter [[w:Subaru Telescope|Subaru Telescope]].<ref name="Meisner_etal_2017b" /><ref name=sciencemag201601/> Unless Planet Nine is observed, its existence is purely conjectural. Several [[w:Planet Nine#Alternative hypotheses|alternative theories]] have been proposed to explain the observed clustering of TNOs.}}

==History==
{{see also|w:Planets beyond Neptune#Orbits of distant objects|w:Detached object#Orbits}}
Following the [[w:discovery of Neptune|discovery of Neptune]] in 1846, there was considerable speculation that another planet might exist beyond its orbit. The best-known of these theories predicted the existence of a distant planet that was influencing the orbits of [[w:Uranus|Uranus]] and [[w:Neptune|Neptune]]. After extensive calculations [[w:Percival Lowell|Percival Lowell]] predicted the possible orbit and location of the hypothetical trans-Neptunian planet and began an extensive search for it in 1906. He called the hypothetical object ''[[w:Planet X|Planet X]]'', a name previously used by Gabriel Dallet.<ref name=grosser>{{cite journal |title=The Search For A Planet Beyond Neptune |author=Morton Grosser |journal=[[Isis (journal)|Isis]] |volume=55 |issue=2 |date=1964 |pages=163–183 |doi=10.1086/349825 |jstor=228182}}</ref><ref name=tombaugh>{{cite journal |title=The Search for the Ninth Planet, Pluto |last=Tombaugh |first=Clyde W. |date=1946 |journal=[[Astronomical Society of the Pacific|Astronomical Society of the Pacific Leaflets]] |volume=5|issue=209 |pages=73–80 |bibcode=1946ASPL....5...73T}}</ref> [[w:Clyde Tombaugh|Clyde Tombaugh]] continued Lowell's search and in 1930 [[w:discovery of Pluto|discovered Pluto]], but it was soon determined to be too small to qualify as Lowell's Planet X.<ref>{{anchor|CITEREFCroswell1997}}{{cite book |title=Planet Quest: The Epic Discovery of Alien Solar Systems |author=Ken Croswell |location=New York |publisher=The Free Press |date=1997 |isbn=978-0-684-83252-4 |pages=57–58}}</ref> After [[w:Voyager 2|Voyager 2]]'s flyby of Neptune in 1989, the difference between Uranus' predicted and observed orbit was determined to have been due to the use of a previously inaccurate mass of Neptune.<ref name="Browne_1993">{{cite news |last1=Browne |first1=Malcolm W. |title=Evidence for Planet X Evaporates in Spotlight of New Research |url=https://www.nytimes.com/1993/06/01/science/evidence-for-planet-x-evaporates-in-spotlight-of-new-research.html |website=New York Times |date=1 June 1993 |accessdate=9 February 2019}}</ref>

Attempts to detect [[w:planets beyond Neptune|planets beyond Neptune]] by indirect means such as orbital perturbation date back to before the discovery of Pluto. Among the first was [[w:George Forbes (scientist)|George Forbes]] who postulated the existence of two trans-Neptunian planets in 1880. One would have an average distance from the Sun, or [[w:semi-major axis|semi-major axis]], of 100 [[w:astronomical unit|astronomical unit]]s (AU), 100 times that of the Earth. The second would have a semimajor axis of 300 AU. His work is considered similar to more recent Planet Nine theories in that the planets would be responsible for a clustering of the orbits of several objects, in this case the [[w:Perihelion and aphelion|aphelion]] distances of periodic [[w:comets|comets]] similar to that of the [[w:Short-period comet|Jupiter-family comets]].<ref name="Millholland_Laughlin_2017"/><ref name="Forbes_1880">{{cite journal |last1=Kirkwood |first1=D. |title=On Comets and Ultra-Neptunian Planets |journal=[[The Observatory (journal)|The Observatory]] |date=1880 |volume=3 |page=439–447 |bibcode=1880Obs.....3..439K |url=http://adsbit.harvard.edu/full/seri/Obs../0003//0000439.000.html}}</ref>

The discovery of [[w:90377 Sedna|Sedna]]'s peculiar orbit in 2004 led to speculation that it had encountered a massive body other than one of the known planets. Sedna's orbit is [[w:detached orbit|detached]], with a [[w:perihelion|perihelion]] distance of 76 AU that is too large to be due to gravitational interactions with Neptune. Several authors proposed that Sedna entered this orbit after encountering an unknown planet on a distant orbit, a member of the [[w:open cluster|open cluster]] that formed with the Sun, or another star that later passed near the Solar System.<ref name=spaceBrown>{{cite web |last=Wall |first=Mike |date=24 August 2011 |title=A Conversation With Pluto's Killer: Q & A With Astronomer Mike Brown |url=http://www.space.com/12711-pluto-killer-mike-brown-dwarf-planets-interview.html |website=[[Space.com]] |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160202232821/http://www.space.com/12711-pluto-killer-mike-brown-dwarf-planets-interview.html |archive-date=2 February 2016 |df=dmy-all}}</ref><ref name=iopsciencesedna>{{cite journal |last1=Brown |first1=Michael E. |last2=Trujillo |first2=Chadwick |last3=Rabinowitz |first3=David |date=2004 |title=Discovery of a Candidate Inner Oort Cloud Planetoid |journal=[[The Astrophysical Journal]] |volume=617 |issue=1 |pages=645–649 |arxiv=astro-ph/0404456 |bibcode=2004ApJ...617..645B |doi=10.1086/422095}}</ref> The announcement in March 2014 of the discovery of a second [[w:sednoid#Known members|sednoid]] with a perihelion distance of 80 AU, {{mpl|2012 VP|113}}, in a similar orbit led to renewed speculation that an unknown super-Earth remained in the distant Solar System.<ref name=guardianSuperEarth>{{cite news |last=Sample |first=Ian |date=26 March 2014 |title=Dwarf Planet Discovery Hints at a Hidden Super Earth in Solar System |url=https://www.theguardian.com/science/2014/mar/26/dwarf-planet-super-earth-solar-system-2012-vp113 |newspaper=[[The Guardian]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160429200749/https://www.theguardian.com/science/2014/mar/26/dwarf-planet-super-earth-solar-system-2012-vp113 |archive-date=29 April 2016 |df=dmy-all}}</ref><ref name=searchformoreSedna/>

At a conference in 2012, Rodney Gomes proposed that an undetected planet was responsible for the orbits of some eTNOs with detached orbits and the large semi-major axis [[w:Centaur (minor planet)|Centaurs]], [[w:small Solar System bodies|small Solar System bodies]] that cross the orbits of the giant planets.<ref name=llmysteries>{{cite web |last=Wolchover |first=Natalie |date=25 May 2012 |title=Planet X? New Evidence of an Unseen Planet at Solar System's Edge |url=http://www.livescience.com/20583-planet-edge-solar-system.html |website=[[LiveScience]] |access-date=7 February 2016 |quote=More work is needed to determine whether Sedna and the other scattered disc objects were sent on their circuitous trips round the Sun by a star that passed by long ago, or by an unseen planet that exists in the solar system right now. Finding and observing the orbits of other distant objects similar to Sedna will add more data points to astronomers' computer models. |archive-url=https://web.archive.org/web/20160130192931/http://www.livescience.com/20583-planet-edge-solar-system.html |archive-date=30 January 2016 |df=dmy-all}}</ref><ref name=nosmokingGun>{{cite web |last=Lovett |first=Richard A. |date=12 May 2012 |title=New Planet Found in Our Solar System? |url=http://news.nationalgeographic.com/news/2012/05/120511-new-planet-solar-system-kuiper-belt-space-science/ |website=[[National Geographic News]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160710060039/http://news.nationalgeographic.com/news/2012/05/120511-new-planet-solar-system-kuiper-belt-space-science/ |archive-date=10 July 2016 |df=dmy-all}}</ref> The proposed Neptune-massed planet would be in a distant (1500 AU), eccentric ([[w:orbital eccentricity|eccentricity]] 0.4), and inclined ([[w:Orbital inclination|inclination]] {{formatnum:40}}°) orbit. Like Planet Nine it would cause the perihelia of objects with semi-major axes greater than 300 AU to oscillate, delivering some into planet-crossing orbits and others into detached orbits like that of Sedna. An article by Gomes, Soares, and Brasser was published in 2015, detailing their arguments.<ref name="Gomes_2015">{{cite journal |last1=Gomes |first1=Rodney |date=2015 |title=The Observation of Large Semi-Major Axis Centaurs: Testing for the Signature of a Planetary-Mass Solar Companion |journal=[[Icarus (journal)|Icarus]] |volume=258 |issue= |pages=37–49 |bibcode=2015Icar..258...37G |doi=10.1016/j.icarus.2015.06.020}}</ref>

In 2014, astronomers [[w:Chad Trujillo|Chad Trujillo]] and [[w:Scott S. Sheppard|Scott S. Sheppard]] noted the similarities in the orbits of Sedna and {{mp|2012 VP|113}} and several other eTNOs. They proposed that an unknown planet in a circular orbit between 200 and 300 AU was perturbing their orbits. Later, in 2015, Raúl and Carlos de la Fuente Marco argued that two massive planets in [[w:orbital resonance|orbital resonance]] were necessary to produce the similarities of so many orbits.<ref name=Sheppard2014/>

==Batygin and Brown hypothesis==
{{fig|1
|Planet nine path in orion2.png
|width=360px
|One hypothetical path through the sky of Planet Nine near [[w:aphelion|aphelion]] crossing [[w:Orion (constellation)|Orion]] west to east with about 2,000 years of motion. It is derived from that employed in the artistic conception on Brown's blog.<ref name="findplanet9">{{cite web|url=http://www.findplanetnine.com/p/blog-page.html|title=Where is Planet Nine?|date=20 January 2016|website=The Search for Planet Nine|type=Blog|archive-url=https://web.archive.org/web/20160130091255/http://www.findplanetnine.com/p/blog-page.html|archive-date=30 January 2016|access-date=}}</ref>
|alt=Starfield with hypothetical path of Planet Nine
}}

In early 2016, [[w:California Institute of Technology|California Institute of Technology]]'s Batygin and Brown described how the similar orbits of six eTNOs could be explained by Planet Nine and proposed a possible orbit for the planet.<ref name=TAJ201601 /> This hypothesis could also explain eTNOs with orbits [[w:perpendicular|perpendicular]] to the [[w:inner planets|inner planets]]<ref name=TAJ201601 /> and others with extreme inclinations,<ref name="Batygin_Brown_high_inc" /> and had been offered as an explanation of the [[w:axial tilt|tilt]] of the Sun's [[w:Rotation around a fixed axis|axis]].<ref name="Gomes_Deienno_Morbidelli_2016" />

===Orbit===

Planet Nine is hypothesized to follow an [[w:elliptical orbit|elliptical orbit]] around the [[w:Sun|Sun]] with an eccentricity of {{val|0.2}} to {{val|0.5}}.  The planet's [[w:semi-major axis|semi-major axis]] is estimated to be {{val|400|u=AU}} to {{val|800|u=AU}},{{efn-ua|A range of semi-major axes extending from 400 AU to 1000 AU produce the observed clustering in simulations.<ref name="bnb20160617" />}} roughly 13 to 26 times the distance from Neptune to the Sun. It would take the planet between 10,000 and 20,000 years to make one full orbit around the Sun.<ref>{{Cite web|url=https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth|title=Planet X|website=NASA Solar System Exploration|access-date=2019-05-14}}</ref> Its inclination to the [[w:ecliptic|ecliptic]], the plane of the Earth's orbit, is projected to be {{val|15|u=°}} to {{val|25|u=°}}.<ref name=P9H2019/> {{efn-ua|''The New Yorker'' put the average orbital distance of Planet Nine into perspective with an apparent allusion to one of the magazine's most famous cartoons, ''[[w:View of the World from 9th Avenue|View of the World from 9th Avenue]]'': "If the Sun were on [[w:Fifth Avenue|Fifth Avenue]] and Earth were one block west, Jupiter would be on the [[w:West Side Highway|West Side Highway]], Pluto would be in [[w:Montclair, New Jersey|Montclair, New Jersey]], and the new planet would be somewhere near [[w:Cleveland|Cleveland]].<ref name=fatty /> "}} The aphelion, or farthest point from the Sun, would be in the general direction of the [[w:constellation|constellation]] of [[w:Taurus (constellation)|Taurus]],<ref>{{cite web |author=Michael E. Brown |date=3 March 2017 |title=Planet Nine |url=https://www.youtube.com/watch?v=v-ktWBtt7sc&t=1147 |at=19:06 |website=[[YouTube]] |access-date=15 March 2017 |archive-url=https://web.archive.org/web/20170406153739/https://www.youtube.com/watch?v=v-ktWBtt7sc&t=1147 |archive-date=6 April 2017 |df=dmy-all |author-link=Michael E. Brown }}</ref> whereas the perihelion, the nearest point to the Sun, would be in the general direction of the southerly areas of [[w:Serpens|Serpens]] (Caput), [[w:Ophiuchus|Ophiuchus]], and [[w:Libra (constellation)|Libra]].<ref name=B-BCharts> {{cite web |last1=Batygin |first1=Konstantin |last2=Brown |first2=Mike |date=20 January 2016 |title=Where is Planet Nine? |url=http://www.findplanetnine.com/p/blog-page.html |at=RA/Dec chart|website=The Search for Planet Nine|publisher=[[Michael E. Brown]] and [[Konstantin Batygin]] |access-date=24 January 2016 |archive-url=https://web.archive.org/web/20160130091255/http://www.findplanetnine.com/p/blog-page.html |archive-date=30 January 2016 }}</ref><ref>{{cite web |last=Lemonick |first=Michael D. |date=20 January 2016 |title=Strong Evidence Suggests a Super Earth Lies beyond Pluto |url=http://www.scientificamerican.com/article/strong-evidence-suggests-a-super-earth-lies-beyond-pluto1/ |at=video |website=[[Scientific American]] |access-date=22 January 2015 |archive-url=https://web.archive.org/web/20160122201344/http://www.scientificamerican.com/article/strong-evidence-suggests-a-super-earth-lies-beyond-pluto1/ |archive-date=22 January 2016 |df=dmy-all }}</ref> Brown thinks that if Planet Nine is confirmed to exist, a [[w:space probe|probe]] could reach it in as little as 20 years by using a [[w:Oberth effect|powered slingshot]] [[w:trajectory|trajectory]] around the Sun.<ref name=newscientist>{{cite web |last1=Becker |first1=Adam |last2=Grossman |first2=Lisa |last3=Aron |first3=Jacob |date=22 January 2016 |title=How Planet Nine May Have Been Exiled to Solar System's Edge |url=https://www.newscientist.com/article/2074961-how-planet-nine-may-have-been-exiled-to-solar-systems-edge/ |website=[[New Scientist]] |accessdate=25 January 2016 |archive-url=https://web.archive.org/web/20160124033216/https://www.newscientist.com/article/2074961-how-planet-nine-may-have-been-exiled-to-solar-systems-edge/ |archive-date=24 January 2016 |df=dmy-all }}</ref>

=== Mass and radius===

The planet is estimated to have 5 to 10 times the mass of Earth and a radius of 2 to 4 times Earth's.<ref name=P9H2019/>  Brown thinks that if Planet Nine exists, its mass is sufficient to [[w:clearing the neighbourhood|clear its orbit]] of large bodies in 4.6 billion years, the age of the Solar System, and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a [[w:Definition of planet#IAU definition|planet by current definitions]].<ref name="lurking" /> Astronomer [[w:Jean-Luc Margot|Jean-Luc Margot]] has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected.<ref>{{cite web |last=Margot |first=Jean-Luc |date=22 January 2016 |title=Would Planet Nine Pass the Planet Test? |url=http://mel.epss.ucla.edu/jlm/epo/planet/nine.html |website=[[University of California at Los Angeles]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160401040546/http://mel.epss.ucla.edu/jlm/epo/planet/nine.html |archive-date=1 April 2016 |df=dmy-all }}</ref><ref name="Margot_2015">{{cite journal |last1=Margot |first1=Jean-Luc |title=A Quantitative Criterion for Defining Planets |journal=[[The Astronomical Journal]] |date=2015 |volume=150 |issue=6 |page=185 |doi=10.1088/0004-6256/150/6/185 |arxiv=1507.06300 |bibcode=2015AJ....150..185M}}</ref>

===Origin===
{{See also|w:Nice model|w:Five-planet Nice model|w:Planetary migration}}

Several possible origins for Planet Nine have been examined including its ejection from the neighborhood of the known giant planets, capture from another star, and ''[[w:in situ|in situ]]'' formation. In their initial article, Batygin and Brown proposed that Planet Nine formed closer to the Sun and was ejected into a distant eccentric orbit following a close encounter with Jupiter or [[w:Saturn|Saturn]] during the nebular epoch.<ref name=TAJ201601 /> The gravity of a nearby star, or drag from the gaseous remnants of the [[w:Solar nebula|Solar nebula]],<ref name="Bromley_Kenyon_2016">{{cite journal |last1=Bromley |first1=Benjamin C. |last2=Kenyon |first2=Scott J. |date=22 July 2016 |title=Making Planet Nine: A Scattered Giant in the Outer Solar System |journal=[[The Astrophysical Journal]] |volume=826 |issue=1 |page=64 |arxiv=1603.08010 |bibcode=2016ApJ...826...64B |doi=10.3847/0004-637X/826/1/64}}</ref> then reduced the eccentricity of its orbit. This raised its perihelion, leaving it in a very wide but stable orbit beyond the influence of the other planets.<ref name=nytimes9thplanet>{{cite news |first=Kenneth |last=Chang |date=20 January 2016 |title=Ninth Planet May Exist Beyond Pluto, Scientists Report |url=https://www.nytimes.com/2016/01/21/science/space/ninth-planet-solar-system-beyond-pluto.html|newspaper=[[The New York Times]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160124105438/http://www.nytimes.com/2016/01/21/science/space/ninth-planet-solar-system-beyond-pluto.html|archive-date=24 January 2016 |df=dmy-all }}</ref><ref name=caltechFAQ>{{cite news |first=Sanden |last=Totten |date=20 January 2016 |title=Caltech Researchers Answer Skeptics' Questions about Planet 9 |url=http://www.scpr.org/news/2016/01/20/56964/caltech-researchers-answer-skeptics-questions-abou/ |publisher=[[89.3 KPCC]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160706171901/http://www.scpr.org/news/2016/01/20/56964/caltech-researchers-answer-skeptics-questions-abou/ |archive-date=6 July 2016 |df=dmy-all }}</ref> The odds of this occurring has been estimated at a few percent.<ref name="Bailey_Fabrycky">{{cite arXiv |last1=Bailey |first1=Nora |last2=Fabrycky |first2=Daniel |title=Stellar Flybys Interrupting Planet-Planet Scattering Generates Oort Planets |date=2018 |eprint=1905.07044|class=astro-ph.EP }}</ref> Had it not been flung into the Solar System's farthest reaches, Planet Nine could have accreted more mass from the [[w:proto-planetary disk|proto-planetary disk]] and developed into the core of a [[w:gas giant|gas giant]].<ref name=lurking /><ref name=dl2018>{{cite book|last=D'Angelo|first=G.|author2=Lissauer, J.J.|chapter=Formation of Giant Planets |bibcode=2018haex.bookE.140D| title=Handbook of Exoplanets |publisher=[[Springer International Publishing AG]] | editor=Deeg H., Belmonte J. |pages=2319–2343|date=2018|doi=10.1007/978-3-319-55333-7_140|arxiv=1806.05649|isbn=978-3-319-55332-0}}</ref> Instead, its growth was halted early, leaving it with a lower mass than Uranus or Neptune.<ref name="Izidoro_etal_2015">{{cite journal |last1=Izidoro |first1=André |last2=Morbidelli |first2=Alessandro |last3=Raymond |first3=Sean N. |last4=Hersant |first4=Franck |last5=Pierens |first5=Arnaud |title=Accretion of Uranus and Neptune from Inward-Migrating Planetary Embryos Blocked by Jupiter and Saturn |journal=[[Astronomy & Astrophysics]] |date=2015 |volume=582 |page=A99 |doi=10.1051/0004-6361/201425525 |arxiv=1506.03029|bibcode=2015A&A...582A..99I }}</ref>

[[w:Dynamical friction|Dynamical friction]] from a massive belt of [[w:planetesimal|planetesimal]]s could also enable Planet Nine's capture in a stable orbit. Recent models propose that a 60–130 Earth mass disk of planetesimals could have formed as the gas was cleared from the outer parts of the proto-planetary disk.<ref name="Carrera_etal_2017">{{cite journal |last1=Carrera |first1=Daniel |last2=Gorti |first2=Uma |last3=Johansen |first3=Anders |last4=Davies |first4=Melvyn B. |title=Planetesimal Formation by the Streaming Instability in a Photoevaporating Disk |journal=[[The Astrophysical Journal]] |date=2017 |volume=839 |issue=1 |page=16 |doi=10.3847/1538-4357/aa6932 |arxiv=1703.07895 |bibcode=2017ApJ...839...16C }}</ref> As Planet Nine passed through this disk its gravity would alter the paths of the individual objects in a way that reduced Planet Nine's velocity relative to it. This would lower the eccentricity of Planet Nine and stabilize its orbit. If this disk had a distant inner edge, 100–200 AU, a planet encountering Neptune would have a 20% chance of being captured in an orbit similar to that proposed for Planet Nine, with the observed clustering more likely if the inner edge is at 200 AU. Unlike the gas nebula, the planetesimal disk is likely to have been long lived, potentially allowing a later capture.<ref name="Eriksson_etal_2017">{{Cite journal|last1=Eriksson|first1=Linn E.J.|last2=Mustill|first2=Alexander J.|last3=Johansen|first3=Anders|title=Circularizing Planet Nine through Dynamical Friction with an Extended, Cold Planetesimal Belt|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=475|issue=4|page=4609|date=2017|arxiv=1710.08295|doi=10.1093/mnras/sty111|bibcode=2018MNRAS.475.4609E}}</ref>

Planet Nine could have been captured from outside the Solar System during a close encounter between the Sun and another star. If a planet was in a distant orbit around this star, [[w:Euler's three-body problem|three-body]] interactions during the encounter could alter the planet's path, leaving it in a stable orbit around the Sun. A planet originating in a system without Jupiter-massed planets could remain in a distant eccentric orbit for a longer time, increasing its chances of capture.<ref name="Mustill_etal_2016">{{cite journal |last1=Mustill |first1=Alexander J. |last2=Raymond |first2=Sean N. |last3=Davies |first3=Melvyn B. |date=21 July 2016 |title=Is there an exoplanet in the Solar System? |journal=[[Monthly Notices of the Royal Astronomical Society: Letters]] |volume=460 |issue=1 |pages=L109–L113 |arxiv=1603.07247 |bibcode=2016MNRAS.460L.109M |doi=10.1093/mnrasl/slw075}}</ref> The wider range of possible orbits would reduce the odds of its capture in a relatively low inclination orbit to 1–2 percent.<ref name="Li_Adams_2016">{{cite journal |last1=Li |first1=Gongjie |last2=Adams |first2=Fred C. |date=2016 |title=Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine |journal=[[The Astrophysical Journal Letters]] |volume=823 |issue=1 |page=L3 |arxiv=1602.08496 |bibcode=2016ApJ...823L...3L |doi=10.3847/2041-8205/823/1/L3}}</ref> This process could also occur with rogue planets, but the likelihood of their capture is much smaller, with only 0.05–0.10% being captured in orbits similar to that proposed for Planet Nine.<ref name="Parker_etal_2017">{{cite journal|last1=Parker|first1=Richard J.|last2=Lichtenberg|first2=Tim|last3=Quanz|first3=Sascha P.|date=2017|title=Was Planet 9 Captured in the Sun's Natal Star-Forming Region?|journal=[[Monthly Notices of the Royal Astronomical Society: Letters]]|volume=472|issue=1|pages=L75–L79|arxiv=1709.00418|doi=10.1093/mnrasl/slx141|bibcode=2017MNRAS.472L..75P}}</ref>

An encounter with another star could also alter the orbit of a distant planet, shifting it from a circular to an eccentric orbit. The ''in situ'' formation of a planet at this distance would require a very massive and extensive disk,<ref name="TAJ201601" /> or the outward drift of solids in a dissipating disk forming a narrow ring from which the planet accreted over a billion years.<ref name="Kenyon_Bromley_2016">{{cite journal |last1=Kenyon |first1=Scott J. |last2=Bromley |first2=Benjamin C. |date=2016 |title=Making Planet Nine: Pebble Accretion at 250–750 AU in a Gravitationally Unstable Ring |journal=[[The Astrophysical Journal]] |volume=825 |issue=1 |page=33 |arxiv=1603.08008 |bibcode=2016ApJ...825...33K |doi=10.3847/0004-637X/825/1/33}}</ref> If a planet formed at such a great distance while the Sun was in its original cluster, the probability of it remaining bound to the Sun in a highly eccentric orbit is roughly 10%.<ref name="Li_Adams_2016" /> A previous article reported that if the massive disk extended beyond 80 AU some objects scattered outward by Jupiter and Saturn would have been left in high inclination (inc > 50°), low eccentricity orbits which have not been observed.<ref name="Kretke_etal_2012">{{cite journal |last1=Kretke |first1=K.A. |last2=Levison |first2=H.F. |last3=Buie |first3=M.W. |last4=Morbidelli |first4=A. |title=A Method to Constrain the Size of the Protosolar Nebula |journal=[[The Astronomical Journal]] |date=2012 |volume=143 |issue=4 |page=91 |doi=10.1088/0004-6256/143/4/91 |arxiv=1202.2343 |bibcode=2012AJ....143...91K}}</ref> An extended disk would also have been subject to gravitational disruption by passing stars and by mass loss due to photoevaporation while the Sun remained in the open cluster where it formed.<ref name="P9H2019" />

===Evidence===
{{TNO}}
The gravitational influence of Planet Nine would explain four peculiarities of the Solar System:<ref name="Brennan_2017">{{cite web |last1=Brennan |first1=Pat |title=The Super-Earth that Came Home for Dinner |url=https://www.jpl.nasa.gov/news/news.php?release=2017-259 |website=[[Jet Propulsion Laboratory]] |accessdate=13 October 2017 |archive-url=https://web.archive.org/web/20171016045431/https://www.jpl.nasa.gov/news/news.php?release=2017-259 |archive-date=16 October 2017 |df=dmy-all }}</ref>
* the clustering of the orbits of eTNOs;
* the high perihelia of objects like [[w:90377 Sedna|90377 Sedna]] that are [[w:detached object|detached]] from Neptune's influence;
* the high inclinations of eTNOs with orbits roughly perpendicular to the orbits of the eight known planets;
* high-inclination [[w:trans-Neptunian object|trans-Neptunian object]]s (TNOs) with semi-major axis less than 100 AU.

Planet Nine was initially proposed to explain the clustering of orbits, via a mechanism that would also explain the high perihelia of objects like Sedna. The evolution of some of these objects into perpendicular orbits was unexpected, but found to match objects previously observed. The orbits of some objects with perpendicular orbits were later found to evolve toward smaller semi-major axes when the other planets were included in simulations. Although other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all four. The gravity of Planet Nine would also increase the inclinations of other objects that cross its orbit, however, which could leave the [[w:scattered disc object|scattered disk objects]],<ref name="Kaib_etal_2019" /> bodies orbiting beyond Neptune with semi-major axes greater than 50 AU, and [[w:Short-period comet|short-period comets]] with a broader inclination distribution than is observed.<ref name="Nesvorny_etal_2017">{{cite journal |last1=Nesvorny |first1=D. |last2=Vokrouhlicky |first2=D. |last3=Dones |first3=L. |last4=Levison |first4=H.F. |last5=Kaib |first5=N. |last6=Morbidelli |first6=A. |journal=[[The Astrophysical Journal]] |volume=845 |issue=1 |page=27 |title=Origin and Evolution of Short-Period Comets |arxiv=1706.07447 |date=2017 |doi=10.3847/1538-4357/aa7cf6 |bibcode=2017ApJ...845...27N}}</ref> Previously Planet Nine was hypothesized to be responsible for the 6 degree tilt of the Sun's axis relative to the orbits of the planets,<ref name="Stirone_sun tilt">{{cite web |last1=Stirone |first1=Shannon |title=Planet Nine May Be Responsible for Tilting the Sun |url=http://www.astronomy.com/news/2016/10/planet-nine-tilting-the-sun |website=[[Astronomy (magazine)|Astronomy]] |accessdate=29 July 2017 |archive-url=https://web.archive.org/web/20170810194731/http://www.astronomy.com/news/2016/10/planet-nine-tilting-the-sun |archive-date=10 August 2017 |df=dmy-all }}</ref> but recent updates to its predicted orbit and mass limit this shift to ~1 degree.<ref name=P9H2019/>

====Observations: Orbital clustering of high perihelion objects====

{{fig|2
|Orbit1.svg
|width=225px
|Diagram illustrating the true anomaly, argument of periapsis, longitude of ascending node, and inclination of a celestial body.
|alt=Orbit of a celestial body is shown as a tilted ellipse intersecting the ecliptic.
}}
The clustering of the orbits of TNOs with large semi-major axes was first described by Trujillo and Sheppard, who noted similarities between the orbits of Sedna and {{mp|2012 VP|113}}. Without the presence of Planet Nine, these orbits should be distributed randomly, without preference for any direction. Upon further analysis, Trujillo and Sheppard observed that the [[w:argument of perihelion|arguments of perihelion]] of 12 TNOs with perihelia greater than {{val|30|u=AU}} and semi-major axes greater than {{val|150|u=AU}} were clustered near zero degrees, meaning that they rise through the ecliptic when they are closest to the Sun. Trujillo and Sheppard proposed that this alignment was caused by a massive unknown planet beyond Neptune via the [[w:Kozai mechanism|Kozai mechanism]].<ref name=Sheppard2014/> For objects with similar semi-major axes the Kozai mechanism would confine their arguments of perihelion to near 0 or 180 degrees. This confinement allows objects with eccentric and inclined orbits to avoid close approaches to the planet because they would cross the plane of the planet's orbit at their closest and farthest points from the Sun, and cross the planet's orbit when they are well above or below its orbit.<ref name="Marcos_2014" /><ref name="KopanyasKozai" /> Trujillo and Sheppard's hypothesis about how the objects would be aligned by the Kozai mechanism has been supplanted by further analysis and evidence.<ref name=TAJ201601 />

Batygin and Brown, looking to refute the mechanism proposed by Trujillo and Sheppard, also examined the orbits of the TNOs with large semi-major axes.<ref name=TAJ201601 /> After eliminating the objects in Trujillo and Sheppard's original analysis that were unstable due to close approaches to Neptune or were affected by Neptune's [[w:mean-motion resonance|mean-motion resonance]]s, Batygin and Brown determined that the arguments of perihelion for the remaining six objects (Sedna, {{mp|2012 VP|113}}, {{mpl-|474640|2004 VN|112}}, {{mpl|2010 GB|174}}, {{mpl-|148209|2000 CR|105}}, and {{mpl-|445473|2010 VZ|98}}) were clustered around {{val|318|8|u=°}}. This finding did not agree with how the Kozai mechanism would tend to align orbits with arguments of perihelion at 0° or 180°.<ref name=TAJ201601 />{{efn-ua|Two types of protection mechanisms are possible:<ref name=KopanyasKozai>{{cite web |first=Barbara |last=Koponyás |date=10 April 2010 |title=Near-Earth Asteroids and the Kozai-Mechanism |url=http://www.univie.ac.at/adg/Conferences/ahw5/talks/KoponysBarbara.pdf.pdf |website=5th Austrian Hungarian Workshop in Vienna |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160314110041/http://www.univie.ac.at/adg/Conferences/ahw5/talks/KoponysBarbara.pdf.pdf |archive-date=14 March 2016 |df=dmy-all }}</ref>

# For bodies whose values of ''a'' and ''e'' are such that they could encounter the planets only near perihelion (or aphelion), such encounters may be prevented by the high inclination and the libration of ''ω'' about 90° or 270° (even when the encounters occur, they do not affect much the minor planet's orbit due to comparatively high relative velocities).
# Another mechanism is viable when at low inclinations when ''ω'' oscillates around 0° or 180° and the minor planet's semi-major axis is close to that of the perturbing planet: in this case the °node crossing occurs always near perihelion and aphelion, far from the planet itself, provided the eccentricity is high enough and the orbit of the planet is almost circular.}}

{{fig|3
|Planet Nine animation.gif
|width=225px
|Orbital correlations among six distant trans-Neptunian objects led to the hypothesis. (See: [[:File:Planet Nine - black background.png|Final frame orbits]].)
|alt=animated diagram zooms out from the orbits of the inner and outer planets to the greatly extended orbits of the outermost objects, which point towards the left of the screen. Planet Nine's hypothetical orbit appears as a broken line
}}

Batygin and Brown also found that the orbits of the six eTNOs with semi-major axes greater than 250 AU and perihelia beyond 30 AU (Sedna, {{mp|2012 VP|113}}, {{mp|2004 VN|112}}, {{mp|2010 GB|174}}, {{mpl|2007 TG|422}}, and {{mpl|2013 RF|98}}) were aligned in space with their perihelia in roughly the same direction, resulting in a clustering of their [[w:longitude of perihelion|longitudes of perihelion]], the location where they make their closest approaches to the Sun. The orbits of the six objects were also tilted with respect to that of the [[w:Ecliptic plane|ecliptic]] and approximately [[w:coplanarity|coplanar]], producing a clustering of their [[w:longitude of ascending node|longitudes of ascending nodes]], the directions where they each rise through the ecliptic. They determined that there was only a 0.007% likelihood that this combination of alignments was due to chance.<ref name=TAJ201601 /><ref name=cbcRadio>{{cite news |last=McDonald |first=Bob |date=24 January 2016 |title=How Did We Miss Planet 9? |url=http://www.cbc.ca/beta/news/technology/planet-9-bob-macdonald-1.3414268 |newspaper=[[CBC News]] |accessdate=18 July 2016 |quote="It's like seeing a disturbance on the surface of water but not knowing what caused it. Perhaps it was a jumping fish, a whale or a seal. Even though you didn't actually see it, you could make an informed guess about the size of the object and its location by the nature of the ripples in the water." |archive-url=https://web.archive.org/web/20160205174831/http://www.cbc.ca/beta/news/technology/planet-9-bob-macdonald-1.3414268 |archive-date=5 February 2016 |df=dmy-all }}</ref><ref name="planetaryblog201601">{{cite web |first=Emily |last=Lakdawalla |date=20 January 2016 |title=Theoretical Evidence for an Undiscovered Super-Earth at the Edge of Our Solar System |url=http://www.planetary.org/blogs/emily-lakdawalla/2016/01200955-theoretical-evidence-for-planet-9.html |website=[[The Planetary Society]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160423222919/http://www.planetary.org/blogs/emily-lakdawalla/2016/01200955-theoretical-evidence-for-planet-9.html |archive-date=23 April 2016 |df=dmy-all }}</ref> These six objects had been discovered by six different surveys on six different telescopes. That made it less likely that the clumping might be due to an observation bias such as pointing a telescope at a particular part of the sky. The observed clustering should be smeared out in a few hundred million years due to the locations of the perihelia and the ascending nodes changing, or [[w:Stellar precession|precessing]], at differing rates due to their varied semi-major axes and eccentricities.{{efn-ua|The precession rate is slower for objects with larger semi-major axes and inclinations and with smaller eccentricities: <math>\dot{\varpi}=\frac{3}{4} \sqrt{\frac{GM}{a^3}} \frac{1}{(1-e^2)^2}\sum_{k=5}^8 \frac{m_k a_k^2}{M a^2}cos^2(i)</math>
where <math> m_k a_k</math> are the masses and semi-major axes of the planets Jupiter through Neptune.}} This indicates that the clustering could not be due to an event in the distant past,<ref name=TAJ201601 /> for example a passing star,<ref name="Hands_etal_2019">{{cite journal |last1=Hands |first1=T. O. |last2=Dehnen |first2=W. |last3=Gration |first3=A. |last4=Stadel |first4=J. |last5=Moore |first5=B. |title=The fate of planetesimal discs in young open clusters: implications for 1I/'Oumuamua, the Kuiper belt, the Oort cloud and more |journal=Monthly Notices of the Royal Astronomical Society |pages=1064 |date=2019 |doi=10.1093/mnras/stz1069 |arxiv=1901.02465|bibcode=2019MNRAS.tmp.1064H }}</ref> and is most likely maintained by the gravitational field of an object orbiting the Sun.<ref name=TAJ201601 />

Two of the six objects ({{mp|2013 RF|98}} and {{mp|2004 VN|112}}) also have very similar orbits and spectra.<ref name="gtcres">{{Cite journal |first=Julia |last=de León |first2=Carlos |last2=de la Fuente Marcos |last3=de la Fuente Marcos |first3=Raúl |year=2017 |title=Visible Spectra of (474640) 2004 VN112-2013 RF98 with OSIRIS at the 10.4 M GTC: Evidence for Binary Dissociation near Aphelion Among the Extreme Trans-Neptunian Objects |journal=[[Monthly Notices of the Royal Astronomical Society: Letters]] |volume=467 |issue=1 |pages=L66–L70 |arxiv=1701.02534 |bibcode=2017MNRAS.467L..66D |doi=10.1093/mnrasl/slx003 |df=dmy-all }}</ref><ref name="binary-ScienceDaily">{{cite web |author=Instituto de Astrofísica de Canarias (IAC)|title=New Data About Two Distant Asteroids Give a Clue to the Possible 'Planet Nine' |url=https://www.sciencedaily.com/releases/2017/02/170221120816.htm |website=[[ScienceDaily]] |accessdate=29 July 2017 |archive-url=https://web.archive.org/web/20170729225604/https://www.sciencedaily.com/releases/2017/02/170221120816.htm |archive-date=29 July 2017 |df=dmy-all }}</ref> This has led to the suggestion that they were a [[w:binary asteroid|binary object]] disrupted near aphelion during an encounter with a distant object. The disruption of a binary would require a relatively close encounter, which becomes less likely at large distances from the Sun.<ref name="Marcos_binary_disruption">{{cite journal |title=Binary Stripping as a Plausible Origin of Correlated Pairs of Extreme Trans-Neptunian Objects |first1=C. |last1=de la Fuente Marcos |first2=R. |last2=de la Fuente Marcos |first3=S.J. |last3=Aarseth |date=1 November 2017 |journal=[[Astrophysics and Space Science]] |volume=362 |issue=11 |page=198 |arxiv=1709.06813 |bibcode=2017Ap&SS.362..198D |doi=10.1007/s10509-017-3181-1 }}</ref>

In a later article Trujillo and Sheppard noted a correlation between the longitude of perihelion and the argument of perihelion of the TNOs with semi-major axes greater than 150 AU. Those with a longitude of perihelion of 0–120° have arguments of perihelion between 280–360°, and those with longitude of perihelion between {{formatnum:180}}° and {{formatnum:340}}° have arguments of perihelion between {{formatnum:0}}° and {{formatnum:40}}°. The statistical significance of this correlation was 99.99%. They suggested that the correlation is due to the orbits of these objects avoiding close approaches to a massive planet by passing above or below its orbit.<ref name="trujillo20160912">{{cite journal |last1=Sheppard, Scott S. |first1=Scott S. |last2=Trujillo |first2=Chadwick |title=New Extreme Trans-Neptunian Objects: Toward a Super-Earth in the Outer Solar System |journal=[[The Astronomical Journal]] |date=2016 |volume=152 |issue=6 |page=221 |doi=10.3847/1538-3881/152/6/221 |arxiv=1608.08772 |bibcode=2016AJ....152..221S }}</ref>

A 2017 article by Carlos and Raul de la Fuente Marcos noted that distribution of the distances to the ascending nodes of the eTNOs, and those of centaurs and comets with large semi-major axes, may be [[w:bimodal|bimodal]]. They suggest it is due to the eTNOs avoiding close approaches to a planet with a semi-major axis of 300–400&nbsp;AU.<ref name=nodes>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |year=2017 |title=Evidence for a Possible Bimodal Distribution of the Nodal Distances of the Extreme Trans-Neptunian Objects: Avoiding a Trans-Plutonian Planet or Just Plain Bias? |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=471 |issue=1 |pages=L61–L65 |arxiv=1706.06981 |bibcode=2017MNRAS.471L..61D |doi=10.1093/mnrasl/slx106 |df=dmy-all }}</ref><ref name="nodes_phys.org">{{cite web |author=Spanish Foundation for Science and Technology (FECYT) |title=New Evidence in Support of the Planet Nine Hypothesis |url=https://phys.org/news/2017-07-evidence-planet-hypothesis.html |website=phys.org |accessdate=29 July 2017 |archive-url=https://web.archive.org/web/20170730014118/https://phys.org/news/2017-07-evidence-planet-hypothesis.html |archive-date=30 July 2017 |df=dmy-all }}</ref>
{{fig|4
 |direction = horizontal
 |color1 = white
 |color2 = white 
 |top1 = 10px
 |top2 = 10px
 |align     = center
 |width1    = 600px
 |width2    = 360px
 |header    = The extreme trans-Neptunian object orbits
 |image1    = Planet nine-etnos now-new3.png
 |image2    = Planet_nine-etnos_now-close-new.png
 |alt1      = Orbits of extreme trans-Neptunian objects and Planet Nine
 |alt2      = Close up of extreme trans-Neptunian objects' and planets' orbits
 |caption1  = Six original and [[w:extreme trans-Neptunian objects|eight additional eTNO objects]] orbits with current positions near their perihelion in purple, with hypothetical Planet Nine orbit in green
 |caption2  = Close up view of 13 eTNO current positions
 |alt       = The orbits of the extreme trans-Neptunian objects are shown in various colors, with Planet Nine's orbit shown in bright green. Most of the orbits are aligned to the right of Planet Nine.
}}
{{-}}

====Simulations: Observed clustering reproduced====

The clustering of the orbits of eTNOs and raising of their perihelia is reproduced in simulations that include Planet Nine. In simulations conducted by Batygin and Brown, swarms of scattered disk objects with semi-major axes up to 550 AU that began with random orientations were sculpted into roughly [[w:collinear|collinear]] and coplanar groups of spatially confined orbits by a massive distant planet in a highly eccentric orbit. This left most of the objects' perihelia pointed in similar directions and the objects' orbits with similar tilts. Many of these objects entered high-perihelion orbits like Sedna and, unexpectedly, some entered perpendicular orbits that Batygin and Brown later noticed had been previously observed.<ref name=TAJ201601 />

In their original analysis Batygin and Brown found that the distribution of the orbits of the first six eTNOs was best reproduced in simulations using a 10 Earth mass{{efn-ua|Batygin and Brown provide an order of magnitude estimate for the mass.
* If ''M'' were equal to 0.1 Earth mass, then the dynamical evolution would proceed at an exceptionally slow rate, and the lifetime of the Solar System would likely be insufficient for the required orbital sculpting to transpire.
* If ''M'' were equal to 1 Earth mass, then long-lived apsidally anti-aligned orbits would indeed occur, but removal of unstable orbits would happen on a much longer timescale than the current evolution of the Solar System. Hence, even though they would show preference for a particular apsidal direction, they would not exhibit true confinement like the data.
* They also note that ''M'' greater than 10 Earth mass would imply a longer semi-major axis.
Hence they estimate that the mass of the object is likely in the range of 5 to 15 Earth mass.}} planet in the following orbit:{{efn-ua|calculated values in parentheses.}}
* [[w:semi-major axis|semi-major axis]] ''a'' ≈ {{val|700|u=AU}} ([[w:orbital period|orbital period]] 700<sup>1.5</sup>=18,520 years)
* [[w:orbital eccentricity|eccentricity]] ''e'' ≈ 0.6, ([[w:perihelion|perihelion]] ≈ {{val|280|u=AU}}, [[w:aphelion|aphelion]] ≈ {{val|1120|u=AU|fmt=commas}})
* [[w:inclination|inclination]] ''i'' ≈ 30° to the [[w:ecliptic|ecliptic]]
* [[w:longitude of the ascending node|longitude of the ascending node]] ''Ω'' ≈ {{val|100||u=°}}.{{efn-ua|The average of longitude of the ascending node for the 6 objects is about 102°. In a blog published later, Batygin and Brown constrained their estimate of the longitude of the ascending node to {{val|94||u=°}}.}}
* [[w:argument of perihelion|argument of perihelion]] ''ω'' ≈ 140° and [[w:longitude of perihelion|longitude of perihelion]] ''ϖ'' = {{val|240|u=°}}<ref name="P9 where are you P1">{{cite web |last1=Brown |first1=Michael E. |title=Planet Nine: Where Are You? (Part 1) |url=http://www.findplanetnine.com/2017/09/planet-nine-where-are-you-part-1.html |website=The Search for Planet Nine |publisher=[[Michael E. Brown]] and [[Konstantin Batygin]]|accessdate=19 October 2017 |archive-url=https://web.archive.org/web/20171020083937/http://www.findplanetnine.com/2017/09/planet-nine-where-are-you-part-1.html |archive-date=20 October 2017 |df=dmy-all }}</ref>

These parameters for Planet Nine produce different simulated effects on TNOs. Objects with semi-major axis greater than 250 AU are strongly anti-aligned with Planet Nine, with perihelia opposite Planet Nine's perihelion. Objects with semi-major axes between 150 AU and 250 AU are weakly aligned with Planet Nine, with perihelia in the same direction as Planet Nine's perihelion. Little effect is found on objects with semi-major axes less than 150 AU.<ref name="bnb20160617"/> The simulations also revealed that objects with semi-major axis greater than {{val|250|u=AU}} could have stable, aligned orbits if they had lower eccentricities. These objects have yet to be observed.<ref name=TAJ201601 />

Other possible orbits for Planet Nine were also examined, with semi-major axes between {{val|400|u=AU}} and {{val|1500|u=AU|}}, eccentricites up to 0.8, and a wide range of inclinations. These orbits yield varied results. Batygin and Brown found that orbits of the eTNOs were more likely to have similar tilts if Planet Nine had a higher inclination, but anti-alignment also decreased.<ref name="bnb20160617" /> Simulations by Becker et al. showed that their orbits were more stable if Planet Nine had a smaller eccentricity, but that anti-alignment was more likely at higher eccentricities.<ref name="Becker_etal_2017">{{cite journal |last1=Becker |first1=Juliette C. |last2=Adams |first2=Fred C. |last3=Khain |first3=Tali |last4=Hamilton |first4=Stephanie J. |last5=Gerdes |first5=David |title=Evaluating the Dynamical Stability of Outer Solar System Objects in the Presence of Planet Nine |journal=[[The Astronomical Journal]] |date=2017 |volume=154 |issue=2 |page=61 |doi=10.3847/1538-3881/aa7aa2 |arxiv=1706.06609 |bibcode=2017AJ....154...61B}}</ref> Lawler et al. found that the population captured in orbital resonances with Planet Nine was smaller if it had a circular orbit, and that fewer objects reached high inclination orbits.<ref name="Lawler_etal_2016">{{cite journal |last1=Lawler |first1=S.M. |last2=Shankman |first2=C. |last3=Kaib |first3=N. |last4=Bannister |first4=M.T. |last5=Gladman |first5=B. |last6=Kavelaars |first6=J.J. |origyear=21 May 2016 |title=Observational Signatures of a Massive Distant Planet on the Scattering Disk |journal=[[The Astronomical Journal]] |date=29 December 2016 |volume=153 |issue=1 |page=33 |bibcode=2017AJ....153...33L |doi=10.3847/1538-3881/153/1/33 |arxiv=1605.06575}}</ref> Investigations by Cáceres et al. showed that the orbits of the eTNOs were better aligned if Planet Nine had a lower perihelion orbit, but its perihelion would need to be higher than 90 AU.<ref name="Caceres_etal_2018">{{cite journal |last1=Cáceres |first1=Jessica |last2=Gomes |first2=Rodney |title=The Influence of Planet 9 on the Orbits of Distant TNOs: The Case for a Low Perihelion Planet |journal=[[The Astronomical Journal]] |volume=156 |issue=4 |page=157 |date=2018 |arxiv=1808.01248|doi=10.3847/1538-3881/aad77a |bibcode=2018AJ....156..157C }}</ref> Later investigations by Batygin et al. found that higher eccentricity orbits reduced the average tilts of the eTNOs orbits.<ref name=P9H2019/> While there are many possible combinations of orbital parameters and masses for Planet Nine, none of the alternative simulations have been better at predicting the observed alignment of objects in the Solar System. The discovery of additional distant Solar System objects would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet. These may also provide further support for, or refutation of, the Planet Nine hypothesis.<ref name=discoveryDeBrief>{{cite magazine |last=Scharping |first=Nathaniel |date=20 January 2016 |title=Planet Nine: A New Addition to the Solar System? |url=http://blogs.discovermagazine.com/d-brief/2016/01/20/planet-9-discovery-shakes-our-understanding-of-the-solar-system/ |magazine=[[Discover (magazine)|Discover]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160716105822/http://blogs.discovermagazine.com/d-brief/2016/01/20/planet-9-discovery-shakes-our-understanding-of-the-solar-system/ |archive-date=16 July 2016 |df=dmy-all }}</ref><ref name=thestarKateAllen>{{cite news |last=Allen |first=Kate |date=20 January 2016 |title=Is a Real Ninth Planet out There Beyond Pluto? |url=https://www.thestar.com/news/world/2016/01/20/is-a-real-ninth-planet-out-there-beyond-pluto.html |newspaper=[[The Toronto Star]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160417232029/http://www.thestar.com/news/world/2016/01/20/is-a-real-ninth-planet-out-there-beyond-pluto.html |archive-date=17 April 2016 |df=dmy-all }}</ref>

Simulations that included the migration of giant planets resulted in a weaker alignment of the eTNOs orbits.<ref name="Nesvorny_etal_2017" /> The direction of alignment also switched, from more aligned to anti-aligned with increasing semi-major axis, and from anti-aligned to aligned with increasing perihelion distance. The latter would result in the sednoids' orbits being oriented opposite most of the other eTNOs.<ref name="Kaib_etal_2019" />

====Dynamics: How Planet Nine modifies the orbits of eTNOs====
{{fig|5
|Secular_evolution_of_eTNOs_induced_by_Planet_Nine.svg
|width=360px
|Long term evolution of eTNOs induced by Planet Nine for objects with semi-major axis of 250 AU.<ref name="Beust_2016" /><ref name="Batygin_Morbidelli_2017" /> Blue: anti-aligned, Red: aligned, Green: metastable, Orange: circulating. Crossing orbits above black line.{{efn-ua|Similar figures in articles by Beust<ref name="Beust_2016" /> and Batygin and Morbidelli<ref name="Batygin_Morbidelli_2017" /> are plots of the Hamiltonian, showing combinations of orbital eccentricities and orientations that have equal energy. If there are no close encounters with Planet Nine, which would change the energy of the orbit, the object's [[w:orbital elements|orbital elements]] remain on one of these curves as the orbits evolve.}}
|alt=the aligned orbits appear as red contour lines on either side of a parabolic black line, while the anti-aligned orbits appear as blue contour lines within the parabola.
}}

{{Main|w:Effects of Planet Nine on trans-Neptunian objects}}

Planet Nine modifies the orbits of eTNOs via a combination of effects. On very long timescales Planet Nine exerts a [[w:torque|torque]] on the orbits of the eTNOs that varies with the alignment of their orbits with Planet Nine's. The resulting exchanges of [[w:angular momentum|angular momentum]] cause the perihelia to rise, placing them in Sedna-like orbits, and later fall, returning them to their original orbits after several hundred million years. The motion of their directions of perihelion also reverses when their eccentricities are small, keeping the objects anti-aligned, see blue curves on diagram, or aligned, red curves. On shorter timescales mean-motion resonances with Planet Nine provides phase protection, which stabilizes their orbits by slightly altering the objects' semi-major axes, keeping their orbits synchronized with Planet Nine's and preventing close approaches. The gravity of Neptune and the other giant planets, and the inclination of Planet Nine's orbit, weaken this protection. This results in a [[w:Chaos theory|chaotic]] variation of semi-major axes as objects hop between resonances, including high order resonances such as 27:17, on million-year timescales.<ref name="Batygin_Morbidelli_2017" /> The mean-motion resonances may not be necessary for the survival of eTNOs if they and Planet Nine are both on inclined orbits.<ref name="Li_etal_2018">{{cite journal |last1=Li |first1=Gongjie |last2=Hadden |first2=Samuel |last3=Payne |first3=Matthew |last4=Holman |first4=Matthew J. |title=The Secular Dynamics of TNOs and Planet Nine Interactions |journal=[[The Astronomical Journal]] |date=2018 |volume=156 |issue=6 |page=263 |doi=10.3847/1538-3881/aae83b |arxiv=1806.06867|bibcode=2018AJ....156..263L }}</ref> The orbital poles of the objects precess around, or circle, the pole of the Solar System's [[w:Laplace plane|Laplace plane]]. At large semi-major axes the Laplace plane is warped toward the plane of Planet Nine's orbit. This causes orbital poles of the eTNOs on average to be tilted toward one side and their longitudes of ascending nodes to be clustered.<ref name="Batygin_Morbidelli_2017">{{cite journal |last1=Batygin |first1=Konstantin |last2=Morbidelli |first2=Alessandro |title=Dynamical Evolution Induced by Planet Nine |journal=[[The Astronomical Journal]] |date=2017 |volume=154 |issue=6 |page=229 |doi=10.3847/1538-3881/aa937c |arxiv=1710.01804 |bibcode=2017AJ....154..229B}}</ref>

====Objects in perpendicular orbits with large semi-major axis====

{{fig|6
|Planet nine TNO orbits.svg
|width=360px
|The orbits of the five objects with high-inclination orbits (nearly perpendicular to the ecliptic) are shown here as cyan ellipses with the hypothetical Planet Nine in orange.
|alt=Planet Nine's orbit is seen pointing towards the top, while the clustered comets are seen towards the bottom.
}}

Planet Nine can deliver eTNOs into orbits roughly perpendicular to the ecliptic.<ref name=extremeTech>{{cite web |last=Hruska |first=Joel |date=20 January 2016 |title=Our Solar System May Contain a Ninth Planet, Far beyond Pluto |url=http://www.extremetech.com/extreme/221595-new-research-suggests-our-solar-system-may-contain-a-ninth-planet-far-beyond-pluto |website=[[ExtremeTech]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160728074926/http://www.extremetech.com/extreme/221595-new-research-suggests-our-solar-system-may-contain-a-ninth-planet-far-beyond-pluto |archive-date=28 July 2016 |df=dmy-all }}</ref><ref name=Siegel /> Several objects with high inclinations, greater than 50°, and large semi-major axes, above 250 AU, have been observed.<ref name=mpc-6>{{cite web |title=MPC list of ''a'' > 250, ''i'' > 40, and ''q'' > 6 |url=http://minorplanetcenter.net/db_search/show_by_properties?semimajor_axis_min=250&inclination_min=40&perihelion_distance_min=6 |website=[[Minor Planet Center]] |access-date=4 February 2016 |archive-url=https://web.archive.org/web/20170802041218/http://minorplanetcenter.net/db_search/show_by_properties?semimajor_axis_min=250&inclination_min=40&perihelion_distance_min=6 |archive-date=2 August 2017 |df=dmy-all }}</ref> These orbits are produced when some low inclination eTNOs enter a [[w:secular resonance|secular resonance]] with Planet Nine upon reaching low eccentricity orbits. The resonance causes their eccentricities and inclinations to increase, delivering the eTNOs into perpendicular orbits with low perihelia where they are more readily observed. The eTNOs then evolve into [[w:Retrograde and prograde motion|retrograde]] orbits with lower eccentricities, after which they pass through a second phase of high eccentricity perpendicular orbits, before returning to low eccentricity and inclination orbits. The secular resonance with Planet Nine involves a [[w:linear combination|linear combination]] of the orbit's arguments and longitudes of perihelion: Δϖ – 2ω. Unlike the Kozai mechanism this resonance causes objects to reach their maximum eccentricities when in nearly perpendicular orbits. In simulations conducted by Batygin and Morbidelli this evolution was relatively common, with 38% of stable objects undergoing it at least once.<ref name="Batygin_Morbidelli_2017" /> The arguments of perihelion of these objects are clustered near or opposite Planet Nine's and their longitudes of ascending node are clustered around 90° in either direction from Planet Nine's when they reach low perihelia.<ref name=TAJ201601 /><ref name="Li_etal_2018" /> This is in rough agreement with observations with the differences attributed to distant encounters with the known giant planets.<ref name=TAJ201601 />

====Orbits of high-inclination objects====

A population of high-inclination TNOs with semi-major axes less than 100 AU may be generated by the combined effects of Planet Nine and the other giant planets. The eTNOs that enter perpendicular orbits have perihelia low enough for their orbits to intersect those of Neptune or the other giant planets. An encounter with one of these planets can lower an eTNO's semi-major axis to below 100 AU, where the object's orbits is no longer controlled by Planet Nine, leaving it in an orbit like {{mpl|2008 KV|42}}. The predicted orbital distribution of the longest lived of these objects is nonuniform. Most would have orbits with perihelia ranging from 5 AU to 35 AU and inclinations below 110 degree; beyond a gap with few objects are would be others with inclinations near 150 degrees and perihelia near 10 AU.<ref name="Batygin_Brown_high_inc">{{cite journal |last1=Batygin |first1=Konstantin |last2=Brown |first2=Michael E. |title=Generation of Highly Inclined Trans-Neptunian Objects by Planet Nine |date=2016 |journal=[[The Astrophysical Journal Letters]] |volume=833 |issue=1 |page=L3 |arxiv=1610.04992 |bibcode=2016ApJ...833L...3B |doi=10.3847/2041-8205/833/1/L3}}</ref> Previously it was proposed that these objects originated in the [[w:Oort Cloud|Oort Cloud]],<ref name="Brasser_etal_2012">{{cite journal |last1=Brasser |first1=R. |last2=Schwamb |first2=M.E. |last3=Lykawka |first3=P.S. |last4=Gomes |first4=R.S. |title=An Oort Cloud Origin for the High-Inclination, High-Perihelion Centaurs |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=2012 |volume=420 |issue=4 |pages=3396–3402 |doi=10.1111/j.1365-2966.2011.20264.x |arxiv=1111.7037 |bibcode=2012MNRAS.420.3396B}}</ref> a theoretical cloud of icy planetesimals surrounding the Sun at distances of 2,000 to 200,000 AU.<ref name="Williams_OortCloud">{{cite web |last1=Williams |first1=Matt |title=What is the Oort Cloud? |url=https://www.universetoday.com/32522/oort-cloud/ |website=Universe Today |date=10 August 2015 |accessdate=25 February 2019}}</ref> In simulations without Planet Nine an insufficient number are produced from the Oort cloud relative to observations, however.<ref name="Kaib_etal_2019" />

====Oort cloud and comets====

Planet Nine would alter the source regions and the inclination distribution of comets. In simulations of the migration of the giant planets described by the [[w:Nice model|Nice model]] fewer objects are captured in the [[w:Oort cloud|Oort cloud]] when Planet Nine is included. Other objects would be captured in a cloud of objects dynamically controlled by Planet Nine. This Planet Nine cloud, made up of the eTNOs and the perpendicular objects, would extend from semi-major axes of 200 AU to 3000 AU and contain roughly 0.3–0.4 Earth masses.<ref name="Nesvorny_etal_2017" /><ref name="Lawler_etal_2016" /> When the perihelia of objects in the Planet Nine cloud drop low enough for them to encounter the other planets some would be scattered into orbits that enter the inner Solar System where they could be observed as comets. If Planet Nine exists these would make up roughly one third of the [[w:Halley-type comet|Halley-type comet]]s. Interactions with Planet Nine would also increase the inclinations of the scattered disk objects that cross its orbit. This could result in more with moderate inclinations of 15–30 degrees than are observed.<ref name="Kaib_etal_2019">{{cite journal |last1=Kaib |first1=Nathan A. |last2=Pike |first2=Rosemary |last3=Lawler |first3=Samantha |last4=Kovalik |first4=Maya |last5=Brown |first5=Christopher |last6=Alexandersen |first6=Mike |last7=Bannister |first7=Michele T. |last8=Gladman |first8=Brett J. |last9=Petit |first9=Jean-Marc |title=OSSOS XVII: Probing the Distant Solar System with Observed Scattering TNOs |date=2019 |arxiv=1905.09286|doi=10.3847/1538-3881/ab2383 }}</ref> The inclinations of the [[w:Short-period comet|Jupiter-family comets]] derived from that population would also have a broader inclination distribution than is observed.<ref name="Nesvorny_etal_2017" /><ref name="Gibbs_IEEE" /> Recent estimates of a smaller mass and eccentricity for Planet Nine would reduce its effect on these inclinations.<ref name="P9H2019" />

===Reception===

Batygin was cautious in interpreting the results of the simulation developed for his and Brown's research article, saying, "Until Planet Nine is caught on camera it does not count as being real. All we have now is an echo."<ref name=TomLevensonAtlantic>{{cite magazine |last=Levenson |first=Thomas |date=25 January 2016 |title=A New Planet or a Red Herring? |url=https://www.theatlantic.com/science/archive/2016/01/a-new-planet-or-a-red-herring/42681 |magazine=[[The Atlantic]] |accessdate=18 July 2016 |quote={{-'}}We plotted the real data on top of the model' Batyagin recalls, and they fell 'exactly where they were supposed to be.' That was, he said, the epiphany. 'It was a dramatic moment. This thing I thought could disprove it turned out to be the strongest evidence for Planet Nine.{{'-}}}}</ref> Brown put the odds for the existence of Planet Nine at about 90%.<ref name=lurking /> [[w:Gregory P. Laughlin|Greg Laughlin]], one of the few researchers who knew in advance about this article, gives an estimate of 68.3%.<ref name=fatty /> Other skeptical scientists demand more data in terms of additional KBOs to be analyzed or final evidence through photographic confirmation.<ref name=vergeSlamDunkandWaterTightEvidence>{{cite news |last=Grush |first=Loren |date=20 January 2016 |title=Our Solar System May Have a Ninth Planet After All – but Not All Evidence Is in (We Still Haven't Seen It Yet) |url=https://www.theverge.com/2016/1/20/10801824/ninth-planet-x-discovered-evidence |newspaper=[[The Verge]] |accessdate=18 July 2016 |quote=The statistics do sound promising, at first. The researchers say there's a 1 in 15,000 chance that the movements of these objects are coincidental and don't indicate a planetary presence at all. ... 'When we usually consider something as clinched and air tight, it usually has odds with a much lower probability of failure than what they have,' says Sara Seager, a planetary scientist at MIT. For a study to be a slam dunk, the odds of failure are usually 1 in 1,744,278. ... But researchers often publish before they get the slam-dunk odds, in order to avoid getting scooped by a competing team, Seager says. Most outside experts agree that the researchers' models are strong. And Neptune was originally detected in a similar fashion—by researching observed anomalies in the movement of Uranus. Additionally, the idea of a large planet at such a distance from the Sun isn't actually that unlikely, according to Bruce Macintosh, a planetary scientist at Stanford University. |archive-url=https://web.archive.org/web/20160729141120/http://www.theverge.com/2016/1/20/10801824/ninth-planet-x-discovered-evidence |archive-date=29 July 2016 |df=dmy-all }}</ref><ref name=thestarKateAllen/><ref name=scantEvidence>{{cite magazine |last=Crocket |first=Christopher |date=31 January 2016 |title=Computer Simulations Heat up Hunt for Planet Nine |url=https://www.sciencenews.org/article/computer-simulations-heat-hunt-planet-nine |magazine=[[Science News]] |access-date=7 February 2016 |quote={{-'}}It's exciting and very compelling work,' says Meg Schwamb, a planetary scientist at Academia Sinica in Taipei, Taiwan. But only six bodies lead the way to the putative planet. 'Whether that's enough is still a question.{{'-}} |archive-url=https://web.archive.org/web/20160206211941/https://www.sciencenews.org/article/computer-simulations-heat-hunt-planet-nine |archive-date=6 February 2016 |df=dmy-all }}</ref> Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.<ref name=pbsBrown>{{cite news |date=22 January 2016 |title=We Can't See This Possible 9th Planet, but We Feel Its Presence |url=https://www.pbs.org/newshour/bb/we-cant-see-this-possible-9th-planet-but-we-feel-its-presence/ |newspaper=[[PBS NewsHour]] |accessdate=18 July 2016 |quote={{-'}}Right now, any good scientist is going to be skeptical, because it's a pretty big claim. And without the final evidence that it's real, there is always that chance that it's not. So, everybody should be skeptical. But I think it's time to mount this search. I mean, we like to think of it as, we have provided the treasure map of where this ninth planet is, and we have done the starting gun, and now it's a race to actually point your telescope at the right spot in the sky and make that discovery of planet nine.' —Mike Brown |archive-url=https://web.archive.org/web/20160722042941/http://www.pbs.org/newshour/bb/we-cant-see-this-possible-9th-planet-but-we-feel-its-presence/ |archive-date=22 July 2016 |df=dmy-all }}</ref>

The Planet Nine hypothesis is supported by several astronomers and academics. [[w:James L. Green|Jim Green]], director of NASA's [[w:Science Mission Directorate|Science Mission Directorate]], said, "the evidence is stronger now than it's been before".<ref name='popsci'>{{cite web |last=Fecht |first=Sarah |date=22 January 2016 |title=Can There Really Be a Planet in Our Solar System That We Don't Know About? |url=http://www.popsci.com/could-there-really-be-9th-planet-that-we-didnt-know-about |website=[[Popular Science]] |accessdate=18 July 2016}}</ref> But Green also cautioned about the possibility of other explanations for the observed motion of distant eTNOs and, quoting [[w:Carl Sagan|Carl Sagan]], he said, "extraordinary claims require extraordinary evidence."<ref name=lurking /> [[w:Massachusetts Institute of Technology|Massachusetts Institute of Technology]] Professor [[w:Thomas Levenson|Tom Levenson]] concluded that, for now, Planet Nine seems the only satisfactory explanation for everything now known about the outer regions of the Solar System.<ref name=TomLevensonAtlantic/> Astronomer [[w:Alessandro Morbidelli (astronomer)|Alessandro Morbidelli]], who reviewed the research article for ''[[w:The Astronomical Journal|The Astronomical Journal]]'', concurred, saying, "I don't see any alternative explanation to that offered by Batygin and Brown."<ref name=fatty/><ref name=lurking/>

Astronomer [[w:Renu Malhotra|Renu Malhotra]] remains agnostic about Planet Nine, but noted that she and her colleagues have found that the orbits of eTNOs seem tilted in a way that is difficult to otherwise explain. "The amount of warp we see is just crazy," she said. "To me, it's the most intriguing evidence for Planet Nine I've run across so far."<ref name=sciamGiantGhost/>

Other authorities have varying degrees of skepticism. American astrophysicist [[w:Ethan Siegel|Ethan Siegel]], who previously speculated that planets may have been ejected from the Solar System during an early dynamical instability, is skeptical of the existence of an undiscovered planet in the Solar System.<ref name=Siegel>{{cite web |authorlink=Ethan Siegel |last1=Siegel |first1=Ethan |date=20 January 2016 |title=Not So Fast: Why There Likely Isn't A Large Planet Beyond Pluto |url=https://www.forbes.com/sites/startswithabang/2016/01/20/not-so-fast-why-there-likely-isnt-a-large-planet-beyond-pluto/#749e30245af4 |website=[[Forbes]] |accessdate=22 January 2016 |archive-url=https://web.archive.org/web/20171014035019/https://www.forbes.com/sites/startswithabang/2016/01/20/not-so-fast-why-there-likely-isnt-a-large-planet-beyond-pluto/#749e30245af4#749e30245af4 |archive-date=14 October 2017 |df=dmy-all }}</ref><ref>{{cite news |authorlink=Ethan Siegel |last1=Siegel |first1=Ethan |date=3 November 2015 |title=Jupiter May Have Ejected a Planet from Our Solar System |url=https://www.forbes.com/sites/startswithabang/2015/11/03/leading-theory-for-how-the-solar-system-formed-just-disproved/ |work=[[Forbes]] |accessdate=22 January 2016 |archive-url=https://web.archive.org/web/20160128152609/http://www.forbes.com/sites/startswithabang/2015/11/03/leading-theory-for-how-the-solar-system-formed-just-disproved/ |archive-date=28 January 2016 |df=dmy-all }}</ref> In a 2018 article discussing a survey that did not find evidence of clustering of the eTNOs' orbits he suggests the previously observed clustering could have been the result of observing bias and claims most scientists think Planet Nine does not exist.<ref>{{cite news |authorlink=Ethan Siegel |last1=Siegel |first1=Ethan |date=14 September 2018 |title=This Is Why Most Scientists Think Planet Nine Doesn't Exist |url=https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/ |work=[[Forbes]] }}</ref> Planetary scientist [[w:Harold F. Levison|Hal Levison]] thinks that the chance of an ejected object ending up in the inner Oort cloud is only about 2%, and speculates that many objects must have been thrown past the Oort cloud if one has entered a stable orbit.<ref name=skyntelescopePlanetX>{{cite magazine |last=Beatty |first=Kelly |date=26 March 2014 |title=New Object Offers Hint of "Planet X" |url=http://www.skyandtelescope.com/astronomy-news/new-object-offers-hint-ofplanet-x/ |magazine=[[Sky & Telescope]] |accessdate=18 July 2016}}</ref>

==Alternative hypotheses==

===Temporary or coincidental clustering===
The results of the Outer Solar System Survey (OSSOS) suggest that the observed clustering is the result of a combination of observing bias and small number statistics. OSSOS, a well-characterized survey of the outer Solar System with known biases, observed eight objects with semi-major axis > 150 AU with orbits oriented in a wide range of directions. After accounting for the observational biases of the survey, no evidence for the arguments of perihelion (ω) clustering identified by Trujillo and Sheppard was seen,{{efn-ua|Of the eight objects with a semi-major axis > 150 AU, OSSOS found three with arguments of perihelion (ω) outside the cluster previously identified by Trujillo and Sheppard (2014):<ref name="Sheppard2014" /> {{mpl|2015 GT|50}}, {{mpl|2015 KH|163}}, and {{mpl|2013 UT|15}}.<ref name="Shankman_etal_2017" />}} and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with being random.<ref name="Shankman_etal_2017">{{cite journal |last1=Shankman |first1=Cory  |display-authors=etal|title=OSSOS. VI. Striking Biases in the Detection of Large Semimajor Axis Trans-Neptunian Objects |journal=[[The Astronomical Journal]] |date=2017 |volume=154 |issue=2 |page=50 |doi=10.3847/1538-3881/aa7aed |arxiv=1706.05348 |bibcode=2017AJ....154...50S|url=https://pure.qub.ac.uk/portal/en/publications/ossos-vi-striking-biases-in-the-detection-of-large-semimajor-axis-transneptunian-objects(027b7a9c-a171-42b4-9449-9355b0a255d4).html |hdl=10150/625487 }}</ref><ref name="Siegel_2018">{{cite web |last1=Siegel |first1=Ethan |title=This Is Why Most Scientists Think Planet Nine Doesn't Exist |url=https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/#cd530456e77c |department=Starts With A Bang |website=[[Forbes]] |accessdate=17 September 2018 |archive-url=https://web.archive.org/web/20180918012518/https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/#cd530456e77c#cd530456e77c |archive-date=18 September 2018 |df=dmy-all }}</ref> This result differed from an analysis of discovery biases in the previously observed eTNOs by Mike Brown. He found that after observation biases were accounted for, the clustering of longitudes of perihelion of 10 known eTNOs would be observed only 1.2% of the time if their actual distribution was uniform. When combined with the odds of the observed clustering of the arguments of perihelion, the probability was 0.025%.<ref name="Brown_discovery biases">{{cite journal |last1=Brown |first1=Michael E. |title=Observational Bias and the Clustering of Distant Eccentric Kuiper Belt Objects |date=2017 |journal=[[The Astronomical Journal]] |volume=154 |issue=2 |page=65 |doi=10.3847/1538-3881/aa79f4 |arxiv=1706.04175 |bibcode=2017AJ....154...65B}}</ref> A later analysis of the discovery biases of 14 eTNOs by Brown and Batygin determined the probability of the observed clustering of the longitudes of perihelion and the orbital pole locations to be 0.2%.<ref name="Brown_Batygin_2019">{{cite journal |last1=Brown |first1=Michael E. |last2=Batygin |first2=Konstantin |title=Orbital Clustering in the Distant Solar System |journal=[[The Astronomical Journal]] |date=2019 |volume=157 |issue=2 |page=62 |doi=10.3847/1538-3881/aaf051 |url=http://web.gps.caltech.edu/~mbrown/papers/ps/clustering.pdf |arxiv=1901.07115|bibcode=2019AJ....157...62B }}</ref>

Simulations of 15 known objects evolving under the influence of Planet Nine also revealed differences from observations. Cory Shankman and his colleagues included Planet Nine in a simulation of many clones (objects with similar orbits) of 15 objects with semi-major axis > 150 AU and perihelion > 30 AU.{{efn-ua|A link to the plots of the orbital evolution of all 15 is included in the arxiv version of the article.}} While they observed alignment of the orbits opposite that of Planet Nine's for the objects with semi-major axis greater than 250 AU, clustering of the arguments of perihelion was not seen. Their simulations also showed that the perihelia of the eTNOs rose and fell smoothly, leaving many with perihelion distances between 50 AU and 70 AU where none had been observed, and predicted that there would be many other unobserved objects.<ref name="Shankman_etal_2016_10" /> These included a large reservoir of high-inclination objects that would have been missed due to most observations being at small inclinations,<ref name="Lawler_etal_2016"/> and a large population of objects with perihelia so distant that they would be too faint to observe. Many of the objects were also ejected from the Solar System after encountering the other giant planets. The large unobserved populations and the loss of many objects led Shankman et al. to estimate that the mass of the original population was tens of Earth masses, requiring that a much larger mass had been ejected during the early Solar System.{{efn-ua|Shankman et al. estimated the mass of this population at tens of Earth masses, and that hundreds to thousands of Earth masses would need to be ejected from the vicinity of the giant planets for this mass to have remained. In the Nice model 20–50 Earth masses is estimated to have been ejected, a significant mass is also ejected from the neighborhoods of the giant planets during their formation.}} Shankman et al. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing eTNOs is a temporary phenomenon that will disappear as more objects are detected.<ref name=sciamGiantGhost>{{cite web |last=Choi |first=Charles Q. |date=25 October 2016 |title=Closing in on a Giant Ghost Planet |url=https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |website=[[Scientific American]] |accessdate=21 March 2017 |archive-url=https://web.archive.org/web/20170728115415/https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |archive-date=28 July 2017 |df=dmy-all }}</ref><ref name="Shankman_etal_2016_10">{{cite journal |last1=Shankman |first1=Cory |last2=Kavelaars |first2=J.J. |last3=Lawler |first3=Samantha |last4=Bannister |first4=Michelle |date=2017 |title=Consequences of a Distant Massive Planet on the Large Semi-Major Axis Trans-Neptunian Objects |journal=[[The Astronomical Journal]] |volume=153 |issue=2 |page=63 |arxiv=1610.04251 |bibcode=2017AJ....153...63S |doi=10.3847/1538-3881/153/2/63|url=https://pure.qub.ac.uk/portal/en/publications/consequences-of-a-distant-massive-planet-on-the-large-semimajor-axis-transneptunian-objects(f94859e2-eab4-46de-8b90-d7028d7bccdd).html }}</ref>

===Inclination instability in a massive disk===

Ann-Marie Madigan and Michael McCourt postulate that an [[w:inclination instability|inclination instability]] in a distant massive belt is responsible for the alignment of the arguments of perihelion of the eTNOs.<ref name="Madigan_McCourt_2016">{{cite journal |last1=Madigan |first1=Ane-Marie |last2=McCourt |first2=Michael |title=A New Inclination Instability Reshapes Keplerian Discs into Cones: Application to the Outer Solar System |journal=[[Monthly Notices of the Royal Astronomical Society: Letters]] |date=2016 |volume=457 |issue=1 |pages=L89–93 |doi=10.1093/mnrasl/slv203 |arxiv=1509.08920 |bibcode=2016MNRAS.457L..89M}}</ref> An inclination instability could occur in a disk of particles with high eccentricity orbits ([[w:Orbital eccentricity|''e'']]&nbsp;>&nbsp;0.6) around a central body, such as the Sun. The self-gravity of this disk would cause its spontaneous organization, increasing the inclinations of the objects and aligning the arguments of perihelion, forming it into a cone above or below the original plane.<ref name="Madigan_etal_2018">{{cite journal |last1=Madigan |first1=Ann-Marie |last2=Zderic |first2=Alexander |last3=McCourt |first3=Michael |last4=Fleisig |first4=Jacob |title=On the Dynamics of the Inclination Instability |journal=[[The Astronomical Journal]] |volume=156 |issue=4 |page=141 |date=2018 |arxiv=1805.03651|doi=10.3847/1538-3881/aad95c |bibcode=2018AJ....156..141M }}</ref> This process would require an extended time and significant mass of the disk, on the order of a billion years for a 1–10&nbsp;Earth-mass disk.<ref name="Madigan_McCourt_2016" /> While an inclination instability could align the arguments of perihelion and raise perihelia, producing detached objects, it would not align the longitudes of perihelion.<ref name="Brown_discovery biases" /> Mike Brown considers Planet Nine a more probable explanation, noting that current surveys have not revealed a large enough scattered-disk to produce an "inclination instability".<ref name=spaceplanet9>{{cite web |last=Wall |first=Mike |date=4 February 2016 |title='Planet Nine'? Cosmic objects' strange orbits may have a different explanation |url=http://www.space.com/31817-planet-nine-existence-question.html |website=[[Space.com]] |quote=We need more mass in the outer solar system," she (Madigan) said. "So it can either come from having more minor planets, and their self-gravity will do this to themselves naturally, or it could be in the form of one single massive planet—a Planet Nine. So it's a really exciting time, and we're going to discover one or the other.|access-date=8 February 2016 |archive-url=https://web.archive.org/web/20160208095024/http://www.space.com/31817-planet-nine-existence-question.html |archive-date=8 February 2016 |df=dmy-all}}</ref><ref name=planet9ornot>{{cite web |last=Snell |first=Jason |date=5 February 2016 |title=This Week in Space: Weird Pluto and No Plan for Mars |url=https://www.yahoo.com/tech/this-week-in-space-weird-pluto-and-no-plan-for-183233179.html |website=[[Yahoo! Tech]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160818145649/https://www.yahoo.com/tech/this-week-in-space-weird-pluto-and-no-plan-for-183233179.html |archive-date=18 August 2016 |df=dmy-all }}</ref> In Nice model simulations of the Solar System that included the self-gravity of the planetesimal disk an inclination instability did not occur. Instead, the simulation produced a rapid precession of the objects' orbits and most of the objects were ejected on too short of a timescale for an inclination instability to occur.<ref name="Fan_Batygin_2017">{{cite journal |last1=Fan |first1=Siteng |last2=Batygin |first2=Konstantin |title=Simulations of the Solar System's early dynamical evolution with a self-gravitating planetesimal disk |journal=[[The Astrophysical Journal]] |volume=851 |issue=2 |pages=L37 |year=2017 |arxiv=1712.07193 |bibcode=2017ApJ...851L..37F |doi=10.3847/2041-8213/aa9f0b}}</ref>

===Shepherding by a massive disk===

Antranik Sefilian and Jihad Touma propose that a massive disk of moderately eccentric TNOs is responsible for the clustering of the longitudes of perihelion of the eTNOs. Their predicted 10 Earth-mass disk would contain TNOs with aligned orbits and eccentricities that increased with their semimajor axes ranging from zero to 0.165. The gravitational effects of this disk would offset the forward precession driven by the giant planets so that the orbital orientations of its individual objects are maintained. The orbits of objects with high eccentricities, such as the observed eTNOs, would be stable and have roughly fixed orientations, or longitudes of perihelion, if their orbits were anti-aligned with this disk.<ref name="Seilian_Touma_2019">{{cite journal |last1=Sefilian |first1=Antranik A. |last2=Touma |first2=Jihad R. |title=Shepherding in a Self-gravitating Disk of Trans-Neptunian Objects |journal=[[The Astronomical Journal]] |date=2019 |volume=157 |issue=2 |page=59 |doi=10.3847/1538-3881/aaf0fc |arxiv=1804.06859|bibcode=2019AJ....157...59S }}</ref> Although Brown thinks the proposed disk could explain the observed clustering of the eTNOs, he finds it implausible that the disk could survive over the age of the Solar System.<ref name="Patel_2019">{{cite web |last1=Patel |first1=Neel V. |title=Planet Nine Might Not Actually Be a Planet |url=https://www.popsci.com/planet-nine-disk-not-planet-tno |website=[[Popular Science]]|date=21 January 2019 |accessdate=21 January 2019}}</ref> Batygin thinks that there is insufficient mass in the Kuiper belt to explain the formation of the disk and asks "why would the protoplanetary disk end near 30 AU and restart beyond 100 AU?"<ref name="Dvorsky_2019">{{cite web |last1=Dvorsky |first1=George |title=Is the Elusive 'Planet Nine' Actually a Massive Ring of Debris in the Outer Solar System? |url=https://gizmodo.com/is-the-elusive-planet-nine-actually-a-massive-ring-of-d-1831961723 |website=[[Gizmodo]] |date= 22 January 2019 |accessdate=23 January 2019}}</ref>
===Planet in lower eccentricity orbit===
{| class="wikitable sortable"  style="text-align:center; float:right;"
|+ Proposed resonant objects for<br/>[[w:Semimajor axis|''a'']]&nbsp;>&nbsp;150&nbsp;AU, [[w:perihelion|''q'']]&nbsp;>&nbsp;40&nbsp;AU<ref name="Malhotra_Volk_Wang"/>
!Body ||[[w:Orbital period|Barycentric period]]<br />(years) ||Ratio
|-
|{{mpl|2013 GP|136}} || 1,830  || 9:1
|-
|{{mpl|2000 CR|105}} ||  3,304  || 5:1
|-
|{{mpl|2012 VP|113}}||  4,300  || 4:1
|-
|{{mpl|2004 VN|112}}||  5,900  || 3:1
|-
|{{mpl|2010 GB|174}}||  6,600  || 5:2
|-
|[[w:90377 Sedna|90377 Sedna]] || ≈ 11,400  || 3:2
|-
! Hypothetical planet ||  ≈ 17,000 || 1:1
|}
The Planet Nine hypothesis includes a set of predictions about the mass and orbit of the planet. An alternative theory predicts a planet with different orbital parameters. Malhotra, Kathryn Volk, and Xianyu Wang have proposed that the four detached objects with the longest orbital periods, those with perihelia beyond {{val|40|u=AU}} and semi-major axes greater than {{val|250|u=AU}}, are in ''n'':1 or ''n'':2 mean-motion resonances with a hypothetical planet. Two other objects with semi-major axes greater than {{val|150|u=AU}} are also potentially in resonance with this planet. Their proposed planet could be on a lower eccentricity, low inclination orbit, with [[w:Orbital eccentricity|eccentricity ''e'']]&nbsp;<&nbsp;0.18 and [[w:Orbital inclination|inclination ''i'']]&nbsp;≈&nbsp;11°. The eccentricity is limited in this case by the requirement that close approaches of {{mp|2010 GB|174}} to the planet are avoided. If the eTNOs are in periodic orbits of the third kind,{{efn-ua|This is often referred (perhaps erroneously) to as Kozai within mean-motion resonance.<ref name="Malhotra_2017">{{cite journal |last1=Malhotra |first1=Renu |title=Prospects for Unseen Planets Beyond Neptune|journal=[[ASP Conference Series]] |volume=513 |page=45 |arxiv=1711.03444 |year=2017 |bibcode=2018ASPC..513...45M}}</ref>}} with their stability enhanced by the libration of their arguments of perihelion, the planet could be in a higher inclination orbit, with [[w:Orbital inclination|''i'']]&nbsp;≈&nbsp;48°. Unlike Batygin and Brown, Malhotra, Volk and Wang do not specify that most of the distant detached objects would have orbits anti-aligned with the massive planet.<ref name="Malhotra_Volk_Wang">{{cite journal |last1=Malhotra |first1=Renu |last2=Volk |first2=Kathryn |last3=Wang |first3=Xianyu |date=2016 |title=Corralling a distant planet with extreme resonant Kuiper belt objects |journal=[[The Astrophysical Journal Letters]] |volume=824 |issue=2 |pages=L22 |arxiv=1603.02196 |bibcode=2016ApJ...824L..22M |doi=10.3847/2041-8205/824/2/L22}}</ref><ref name="Malhotra_TED_2017">{{cite web |last1=Malhotra |first1=Renu |title=The search for Planet Nine |date=15 April 2018 |url=https://www.youtube.com/watch?v=MptrypvBTag |website=[[YouTube]] |accessdate=18 Jan 2019 |df=dmy-all}}</ref>

===Alignment due to the Kozai mechanism===

Trujillo and Sheppard argued in 2014 that a massive planet in a circular orbit with an average distance between {{val|200|u=AU}} and {{val|300|u=AU}} was responsible for the clustering of the arguments of perihelion of twelve TNOs with large semi-major axes. Trujillo and Sheppard identified a clustering near zero degrees of the arguments of perihelion of the orbits of twelve TNOs with perihelia greater than {{val|30|u=AU}} and semi-major axes greater than {{val|150|u=AU}}.<ref name=TAJ201601 /><ref name=Sheppard2014/> After numerical simulations showed that the arguments of perihelion should circulate at varying rates, leaving them randomized after billions of years, they suggested that a massive planet in a circular orbit at a few hundred astronomical units was responsible for this clustering.<ref name=Sheppard2014/><ref name=sciencenews>{{cite web |last=Crocket |first=Christopher |date=14 November 2014 |title=A Distant Planet May Lurk Far Beyond Neptune |url=https://www.sciencenews.org/article/distant-planet-may-lurk-far-beyond-neptune |website=[[Science News]] |access-date=7 February 2015 |archive-url=https://web.archive.org/web/20150415165100/https://www.sciencenews.org/article/distant-planet-may-lurk-far-beyond-neptune/ |archive-date=15 April 2015 }}</ref> This massive planet would cause the arguments of perihelion of the TNOs to librate about 0° or 180° via the Kozai mechanism so that their orbits crossed the plane of the planet's orbit near perihelion and aphelion, the closest and farthest points from the planet.<ref name="Sheppard2014">{{cite journal|last1=Trujillo|first1=Chadwick A.|author-link=Chad Trujillo|last2=Sheppard|first2=Scott S.|author-link2=Scott S. Sheppard|date=2014|title=A Sedna-like Body with a Perihelion of 80 Astronomical Units|url=http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf|journal=[[Nature (journal)|Nature]]|volume=507|issue=7493|pages=471–474|bibcode=2014Natur.507..471T|doi=10.1038/nature13156|pmid=24670765|url-access=subscription|via=|accessdate=20 January 2016|archive-url=https://web.archive.org/web/20141216183818/http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf|archive-date=16 December 2014|df=dmy-all}}</ref><ref name="Marcos_2014" /> In numerical simulations including a 2–15 Earth mass body in a circular low-inclination orbit between {{val|200|u=AU}} and {{val|300|u=AU}} the arguments of perihelia of Sedna and {{mp|2012 VP|113}} librated around 0° for billions of years (although the lower perihelion objects did not) and underwent periods of libration with a Neptune mass object in a high inclination orbit at 1,500 AU.<ref name="Sheppard2014" /> Another process such as a passing star would be required to account for the absence of objects with arguments of perihelion near 180°.<ref name=TAJ201601 />{{efn-ua|Assuming that the orbital elements of these objects have not changed, Jílková et al. proposed an encounter with a passing star might have helped acquire these objects – dubbed sednitos ([[w:eTNO|eTNO]]s with {{nowrap|''q'' > 30}} and {{nowrap|''a'' > 150}}) by them. They also predicted that the ''sednitos'' region is populated by 930 planetesimals and the inner Oort Cloud acquired ∼440 planetesimals through the same encounter.<ref name=JilkovaSednito>{{cite journal |last1=Jílková |first1=Lucie |last2=Portegies Zwart |first2=Simon |last3=Pijloo |first3=Tjibaria |last4=Hammer |first4=Michael |date=2015 |title=How Sedna and Family Were Captured in a Close Encounter with a Solar Sibling |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=453 |issue=3 |pages=3157–3162 |arxiv=1506.03105 |bibcode=2015MNRAS.453.3157J |doi=10.1093/mnras/stv1803}}</ref><ref name=stealingsedna>{{cite web |last=Dickinson |first=David |date=6 August 2015 |title=Stealing Sedna |url=http://www.universetoday.com/121637/stealing-sedna/ |website=[[Universe Today]] |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160207202836/http://www.universetoday.com/121637/stealing-sedna/ |archive-date=7 February 2016 |df=dmy-all }}</ref>}}

These simulations showed the basic idea of how a single large planet can shepherd the smaller TNOs into similar types of orbits. They were basic proof of concept simulations that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have.<ref name=sciencenews/> Thus they did not fully formulate a model that successfully incorporated all the clustering of the eTNOs with an orbit for the planet.<ref name=TAJ201601 /> But they were the first to notice there was a clustering in the orbits of TNOs and that the most likely reason was from an unknown massive distant planet. Their work is very similar to how [[w:Alexis Bouvard|Alexis Bouvard]] noticed Uranus' motion was peculiar and suggested that it was likely gravitational forces from an unknown 8th planet, which led to the discovery of Neptune.<ref name="Bouvard">{{cite web |last1=O'Connor |first1=J.J. |last2=Robertson |first2=E.F. |title=Alexis Bouvard |url=http://www-history.mcs.st-and.ac.uk/Biographies/Bouvard.html |website=[[MacTutor History of Mathematics archive]] |access-date=20 October 2017 |archive-url=https://web.archive.org/web/20171025124202/http://www-history.mcs.st-and.ac.uk/Biographies/Bouvard.html |archive-date=25 October 2017 |df=dmy-all }}</ref>

Raúl and Carlos de la Fuente Marcos proposed a similar model but with two distant planets in resonance.<ref name="Marcos_2014">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |year=2014 |title=Extreme Trans-Neptunian Objects and the Kozai Mechanism: Signalling the Presence of Trans-Plutonian Planets |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=443 |issue=1 |pages=L59–L63 |arxiv=1406.0715 |bibcode=2014MNRAS.443L..59D |doi=10.1093/mnrasl/slu084}}</ref><ref name=time2planets>{{cite magazine |last=Lemonick |first=Michael D. |date=19 January 2015 |title=There May Be 'Super Earths' at the Edge of Our Solar System |url=http://time.com/3673527/super-earths-possibly-discovered/ |magazine=[[Time (magazine)|Time]] |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160128125505/http://time.com/3673527/super-earths-possibly-discovered/ |archive-date=28 January 2016 |df=dmy-all }}</ref> An analysis by Carlos and Raúl de la Fuente Marcos with [[w:Sverre Aarseth|Sverre J. Aarseth]] confirmed that the observed alignment of the arguments of perihelion could not be due to observational bias. They speculated that instead it was caused by an object with a mass between that of Mars and Saturn that orbited at some {{val|200|u=AU}} from the Sun. Like Trujillo and Sheppard they theorized that the TNOs are kept bunched together by a Kozai mechanism and compared their behavior to that of [[w:96P/Machholz|Comet 96P/Machholz]] under the influence of [[w:Jupiter|Jupiter]].<ref name=mnrasflippingcomets>{{cite journal
 |last1        = de la Fuente Marcos
 |first1       = Carlos
 |last2        = de la Fuente Marcos
 |first2       = Raúl
 |last3        = Aarseth
 |first3       = Sverre J.
 |year         = 2015
 |title        = Flipping Minor Bodies: What Comet 96P/Machholz 1 Can Tell Us About the Orbital Evolution of Extreme Trans-Neptunian Objects and the Production of Near-Earth Objects on Retrograde Orbits
 |journal      = [[Monthly Notices of the Royal Astronomical Society]]
 |volume       = 446
 |issue        = 2
 |pages        = 1867–187
 |arxiv        = 1410.6307
 |bibcode      = 2015MNRAS.446.1867D
 |doi          = 10.1093/mnras/stu2230
 |df           = dmy-all
}}</ref> They also struggled to explain the orbital alignment using a model with only one unknown planet, and therefore suggested that this planet is itself in resonance with a more-massive world about {{val|250|u=AU}} from the Sun.<ref name=sciencenews/><ref name=universetoday2planets>{{cite web |last=Atkinson |first=Nancy |date=15 January 2015 |title=Astronomers Are Predicting at Least Two More Large Planets in the Solar System |url=http://www.universetoday.com/118252/astronomers-are-predicting-at-least-two-more-large-planets-in-the-solar-system/ |website=[[Universe Today]] |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160206145105/http://www.universetoday.com/118252/astronomers-are-predicting-at-least-two-more-large-planets-in-the-solar-system/ |archive-date=6 February 2016 |df=dmy-all }}</ref> In their article, Brown and Batygin noted that alignment of arguments of perihelion near 0° or 180° via the Kozai mechanism requires a ratio of the semi-major axes nearly equal to one, indicating that multiple planets with orbits tuned to the data set would be required, making this explanation too unwieldy.<ref name=TAJ201601 />

==Detection attempts==

===Visibility and location===
Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings.<ref name=lurking /> It is expected to have an [[w:apparent magnitude|apparent magnitude]] fainter than 22, making it at least 600&nbsp;times fainter than [[w:Pluto|Pluto]].<ref name="bnb20160617" />{{efn-ua|The 8&nbsp;meter [[w:Subaru Telescope|Subaru Telescope]] has achieved a 27.7&nbsp;magnitude photographic limit with a ten-hour exposure,<ref>{{cite magazine |date=24 July 2006 |title=What is the faintest object imaged by ground-based telescopes? |url=http://www.skyandtelescope.com/astronomy-resources/astronomy-questions-answers/what-is-the-faintest-object-imaged-by-ground-based-telescopes/ |magazine=[[Sky & Telescope]] |accessdate=18 July 2016}}</ref> which is about 100&nbsp;times dimmer than Planet Nine is expected to be. For comparison, the [[w:Hubble Space Telescope|Hubble Space Telescope]] has detected objects as faint as 31st&nbsp;magnitude with an exposure of about 2&nbsp;million seconds (555&nbsp;hours) during [[w:Hubble Ultra Deep Field|Hubble Ultra Deep Field]] photography.<ref>{{cite web |last1=Illingworth |first1=G. |last2=Magee |first2=D. |last3=Oesch |first3=P. |last4=Bouwens |first4=R. |date=25 September 2012 |title=Hubble goes to the extreme to assemble the deepest ever view of the universe |url=https://www.spacetelescope.org/news/heic1214/ |website=[[Hubble Space Telescope]] |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160201104727/https://www.spacetelescope.org/news/heic1214/ |archive-date=1 February 2016 |df=dmy-all }}</ref> Hubble’s field of view is very narrow, as is the [[w:Keck Observatory|Keck Observatory]]’s [[w:Large Binocular Telescope|Large Binocular Telescope]].<ref name=sciencemag201601>{{cite journal |first=Eric |last=Hand |date=20 January 2016 |title=Astronomers say a Neptune-sized planet lurks beyond Pluto |url=http://www.sciencemag.org/news/2016/01/feature-astronomers-say-neptune-sized-planet-lurks-unseen-solar-system |journal=[[Science (journal)|Science]] |doi=10.1126/science.aae0237 |accessdate=20 January 2016 |archive-url=https://web.archive.org/web/20160120170200/http://www.sciencemag.org/news/2016/01/feature-astronomers-say-neptune-sized-planet-lurks-unseen-solar-system |archive-date=20 January 2016 |df=dmy-all }}</ref> Brown hopes to make a request for use of the ''[[w:Hubble Space Telescope|Hubble Space Telescope]]'' the day the planet is spotted.<ref>{{cite web |author=Deep Astronomy |date=19 February 2016 |title=Ninth Planet Beyond Neptune? |website=[[YouTube]] |url=https://www.youtube.com/watch?v=1jPxZQoVVZo&t=46m57s |at=46:57}}</ref>}} If Planet Nine exists and is close to perihelion, astronomers could identify it based on existing images. At aphelion, the largest telescopes would be required, but if the planet is currently located in between, many [[w:observatory|observatories]] could spot Planet Nine.<ref name=CaltechJan2016 /> Statistically, the planet is more likely to be close to its aphelion at a distance greater than 600&nbsp;AU.<ref name="NatGeo201601" /> This is because objects move more slowly when near their aphelion, in accordance with [[w:Kepler's second law|Kepler's second law]]. A 2019 study estimated that Planet Nine, if it exists, may be smaller and closer than originally thought. This would make the hypothetical planet brighter and easier to spot, with an apparent magnitude of &nbsp;21–22.<ref name="P9H2019" /><ref name="more support">{{cite web| url=https://phys.org/news/2019-02-planet.html |title=More support for Planet Nine| publisher=Phys.org| date=27 February 2019 |accessdate=26 June 2019}}</ref> According to [[w:University of Michigan|University of Michigan]] professor Fred Adams, within the next 10 to 15&nbsp;years, Planet Nine will either be observable or enough data will have been gathered to rule out its existence.<ref>{{Cite web |url=https://www.techradar.com/news/are-we-getting-closer-to-finding-planet-nine |title=Are we getting closer to finding 'Planet Nine'? |department=Future tech |first=Jamie |last=Carter |date=2019-03-25 |website=TechRadar |language=en |access-date=2019-05-14 |df=dmy-all}}</ref><ref name="boost">{{cite web|url=https://earthsky.org/space/review-paper-physics-reports-planet-9-feb-2019|title=Planet 9 hypothesis gets a boost|author=Paul Scott Anderson|publisher=EarthSky|date=3 March 2019|accessdate=26 June 2019}}</ref>

===Searches of existing data===

The search of [[w:Star catalogue|databases of stellar objects]] by Batygin and Brown has already excluded much of the sky along Planet Nine's predicted orbit.  The remaining regions include the direction of its aphelion, where it would be too faint to be spotted by these surveys, and near the plane of the [[w:Milky Way|Milky Way]], where it would be difficult to distinguish from the numerous stars.<ref name=B-BCharts/> This search included the archival data from the [[w:Catalina Sky Survey|Catalina Sky Survey]] to magnitude c. 19, [[w:Pan-STARRS|Pan-STARRS]] to magnitude 21.5, and infrared data from the [[w:Wide-field Infrared Survey Explorer|Wide-field Infrared Survey Explorer]] (WISE) satellite.<ref name=findplanet9 /<ref name="bnb20160617" /><ref name=B-BCharts/> They have more recently also searched the first year data release from the [[w:Zwicky Transient Facility|Zwicky Transient Facility]] without identifying Planet Nine.<ref name="Brown_ZTF">{{cite web |last1=Brown |first1=Michael |title=@plutokiller |url=https://twitter.com/plutokiller/status/1137082196953460736 |website=[[Cyberculture/Social Media/Twitter|Twitter]] |accessdate=7 June 2019}}</ref>

Other researchers have been conducting searches of existing data. David Gerdes, who helped develop the camera used in the [[w:Dark Energy Survey|Dark Energy Survey]], claims that software designed to identify distant Solar System objects such as {{mpl|2014 UZ|224}} could find Planet Nine if it was imaged as part of that survey, which covered a quarter of the southern sky.<ref name=npr2014UZ224>{{cite web
 |title=A Friend For Pluto: Astronomers Find New Dwarf Planet In Our Solar System
 |website=[[NPR]]
 |first=Joe
 |last=Palka
 |url=https://www.npr.org/sections/thetwo-way/2016/10/11/497071139/a-friend-for-pluto-astronomers-find-new-dwarf-planet-in-our-solar-system
 |access-date=5 April 2018
 |archive-url=https://web.archive.org/web/20180405032925/https://www.npr.org/sections/thetwo-way/2016/10/11/497071139/a-friend-for-pluto-astronomers-find-new-dwarf-planet-in-our-solar-system
 |archive-date=5 April 2018
  |df=dmy-all
}}</ref><ref name=Shannon_NewSci>{{cite web |last=Hall |first=Shannon |date=20 April 2016 |title=We Are Closing in on Possible Whereabouts of Planet Nine |url=https://www.newscientist.com/article/2084924-we-are-closing-in-on-possible-whereabouts-of-planet-nine/ |website=[[New Scientist]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160617082953/https://www.newscientist.com/article/2084924-we-are-closing-in-on-possible-whereabouts-of-planet-nine/ |archive-date=17 June 2016 |df=dmy-all }}</ref> Michael Medford and Danny Goldstein, graduate students at the [[w:University of California, Berkeley|University of California, Berkeley]], are also examining archived data using a technique that combines images taken at different times. Using a [[w:supercomputer|supercomputer]] they will offset the images to account for the calculated motion of Planet Nine, allowing many faint images of a faint moving object to be combined to produce a brighter image.<ref name="Gibbs_IEEE">{{cite web |last1=Gibbs |first1=W. Wayt |title=Is There a Giant Planet Lurking Beyond Pluto? |url=http://spectrum.ieee.org/aerospace/satellites/is-there-a-giant-planet-lurking-beyond-pluto |website=[[IEEE Spectrum]] |accessdate=1 August 2017 |archive-url=https://web.archive.org/web/20170801005216/http://spectrum.ieee.org/aerospace/satellites/is-there-a-giant-planet-lurking-beyond-pluto |archive-date=1 August 2017 |df=dmy-all }}</ref> A search combining multiple images collected by WISE and NEOWISE data has also been conducted without detecting Planet Nine. This search covered regions of the sky away from the galactic plane at the "W1" wavelength (the 3.4&nbsp;μm wavelength used by WISE) and is estimated to be able to detect a 10-Earth mass object out to 800–900 AU.<ref name="Meisner_etal_2017b" /><ref name="Meisner_etal_2017a">{{cite journal |last1=Meisner |first1=Aaron M. |last2=Bromley |first2=Benjamin B. |last3=Nugent |first3=Peter E. |last4=Schlegel |first4=David J |last5=Kenyon |first5=Scott J. |last6=Schlafly |first6=Edward F. |last7=Dawson |first7=Kyle S. |title=Searching for Planet Nine with Coadded WISE and NEOWISE-Reactivation Images |journal=[[The Astronomical Journal]] |date=2016 |volume=153 |issue=2 |page=65 |doi=10.3847/1538-3881/153/2/65 |arxiv=1611.00015 |bibcode=2017AJ....153...65M}}</ref>

===Ongoing searches===

Because the planet is predicted to be visible in the [[w:Northern Hemisphere|Northern Hemisphere]], the primary search is expected to be carried out using the [[w:Subaru Telescope|Subaru Telescope]], which has both an [[w:Objective (optics)|aperture]] large enough to see faint objects and a wide field of view to shorten the search.<ref name=searchformoreSedna>{{cite web |first=Nicole |last=Mortillaro |date=9 February 2016 |title=Meet Mike Brown: Pluto Killer and the Man Who Brought Us Planet 9 |url=http://globalnews.ca/news/2500010/meet-mike-brown-pluto-killer-and-the-man-who-brought-us-planet-9/ |website=[[Global News]] |quote={{-'}}It was that search for more objects like Sedna ... led to the realization ... that they're all being pulled off in one direction by something. And that's what finally led us down the hole that there must be a big planet out there.' —Mike Brown |access-date=10 February 2016 |archive-url=https://web.archive.org/web/20160210095238/http://globalnews.ca/news/2500010/meet-mike-brown-pluto-killer-and-the-man-who-brought-us-planet-9/ |archive-date=10 February 2016 |df=dmy-all }}</ref> Two teams of astronomers—Batygin and Brown, as well as Trujillo and Sheppard—are undertaking this search together, and both teams expect the search to take up to five years.<ref name=sciencemag201601/><ref name=SpaceCom>{{cite web |last1=Wall |first1=Mike |date=21 January 2016 |title=How Astronomers Could Actually See 'Planet Nine' |url=http://www.space.com/31677-astronomers-could-see-planet-nine.html |website=[[Space.com]] |accessdate=24 January 2016 |archive-url=https://web.archive.org/web/20160123220605/http://www.space.com/31677-astronomers-could-see-planet-nine.html |archive-date=23 January 2016 |df=dmy-all }}</ref> Brown and Batygin initially narrowed the search for Planet Nine down to roughly 2,000 [[w:square degree|square degree]]s of sky near [[w:Orion (constellation)|Orion]], a swath of space that Batygin thinks could be covered in about 20 nights by the Subaru Telescope.<ref name=crockettnewclues>{{cite magazine |first=Christopher |last=Crockett |date=5 July 2016 |title=New Clues in Search for Planet Nine |url=https://www.sciencenews.org/article/new-clues-search-planet-nine |magazine=[[Science News]] |accessdate=6 July 2016 |archive-url=https://web.archive.org/web/20160705130303/https://www.sciencenews.org/article/new-clues-search-planet-nine |archive-date=5 July 2016 |df=dmy-all }}</ref> Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky.<ref>{{cite web |last=Choi |first=Charles C. |url=https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |title=Closing in on a Giant Ghost Planet |website=[[Scientific American]] |date=25 October 2016 |accessdate=26 October 2016 |archive-url=https://web.archive.org/web/20161026023621/https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |archive-date=26 October 2016 |df=dmy-all }}</ref> In December 2018, they spent 4 half–nights and 3 full nights observing with the Subaru Telescope.<ref name="Stirone_2019">{{cite web |last1=Stirone |first1=Shannon |title=The Hunt for Planet Nine |url=https://longreads.com/2019/01/22/the-hunt-for-planet-nine/ |website=Longreads |date=22 January 2019 | accessdate=22 January 2019}}</ref>

===Radiation===
Although a distant planet such as Planet Nine would reflect little light, due to its large mass it would still be radiating the heat from its formation as it cools. At its estimated temperature of {{convert|47|K|C}} the peak of its emissions would be at [[w:infrared|infrared]] wavelengths.<ref name="Linder_Mordasini_2016">{{cite journal |last1=Linder |first1=Esther F. |last2=Mordasini |first2=Christoph |date=2016 |title=Evolution and Magnitudes of Candidate Planet Nine |journal=[[Astronomy & Astrophysics]] |volume=589 |issue= 134|pages=A134 |arxiv=1602.07465 |bibcode=2016A&A...589A.134L |doi=10.1051/0004-6361/201628350}}</ref> This radiation signature could be detected by Earth-based [[w:submillimeter telescope|submillimeter telescope]]s, such as [[w:Atacama Large Millimeter Array|ALMA]],<ref name=discoverPerspective>{{cite magazine |last=Powel |first=Corey S. |date=22 January 2016 |title=A Little Perspective on the New "9th&nbsp;Planet" (and the 10th, and the 11th) |url=http://blogs.discovermagazine.com/outthere/2016/01/22/new-9th-planet/#.VqYOnZp97iw |magazine=[[Discover (magazine)|Discover]] |accessdate=18 July 2016 |archive-url=https://web.archive.org/web/20160714182831/http://blogs.discovermagazine.com/outthere/2016/01/22/new-9th-planet/#.VqYOnZp97iw#.VqYOnZp97iw |archive-date=14 July 2016 |df=dmy-all }}</ref> and a search could be conducted by [[w:cosmic microwave background|cosmic microwave background]] experiments operating at [[w:Microwave#Radio astronomy|mm wavelengths]].<ref>{{cite journal |last1=Cowan |first1=Nicolas B. |last2=Holder |first2=Gil |last3=Kaib |first3=Nathan A. |date=2016 |title=Cosmologists in Search of Planet Nine: the Case for CMB Experiments |journal=[[The Astrophysical Journal Letters]] |volume=822 |issue=1 |pages=L2 |arxiv=1602.05963 |bibcode=2016ApJ...822L...2C |doi=10.3847/2041-8205/822/1/L2}}</ref><ref name="Aron_2016">{{cite web |last1=Aron |first1=Jacob |date=24 February 2016 |title=Planet Nine Hunters Enlist Big Bang Telescopes and Saturn Probe |url=https://www.newscientist.com/article/2078757-planet-nine-hunters-enlist-big-bang-telescopes-and-saturn-probe/ |website=[[New Scientist]] |accessdate=27 February 2016 |archive-url=https://web.archive.org/web/20160225105639/https://www.newscientist.com/article/2078757-planet-nine-hunters-enlist-big-bang-telescopes-and-saturn-probe/ |archive-date=25 February 2016 |df=dmy-all}}</ref><ref>{{cite news |title=Is there a mysterious Planet Nine lurking in our Solar system beyond Neptune? |newspaper=[[Washington Post]] |first=Charlie |last=Wood |date=2 Sep 2018 |url=https://www.washingtonpost.com/national/health-science/is-there-a-mysterious-planet-nine-lurking-in-our-solar-system-beyond-neptune/2018/08/31/1957c8ca-a495-11e8-8fac-12e98c13528d_story.html |access-date=17 Jan 2019 |archive-url=https://web.archive.org/web/20180902220558/https://www.washingtonpost.com/national/health-science/is-there-a-mysterious-planet-nine-lurking-in-our-solar-system-beyond-neptune/2018/08/31/1957c8ca-a495-11e8-8fac-12e98c13528d_story.html |archive-date=2 September 2018 |df=dmy-all}}</ref>{{efn-ua|It is estimated that to find Planet Nine, telescopes that can resolve a 30&nbsp;[[w:Jansky|mJy]] point source are needed, and that can also resolve an annual [[w:stellar parallax|parallax motion]] of ~5&nbsp;[[w:arcminute|arcminute]]s.<ref name=aasnovacmb>{{cite web |last=Kohler |first=Susanna |date=25 April 2016 |title=Can CMB experiments find Planet Nine? |url=http://aasnova.org/2016/04/25/can-cmb-experiments-find-planet-nine/ |website=AAS Nova |publisher=[[American Astronomical Society]] |accessdate=29 April 2016 |archive-url=https://web.archive.org/web/20160531185715/http://aasnova.org/2016/04/25/can-cmb-experiments-find-planet-nine/ |archive-date=31 May 2016 |df=dmy-all }}</ref>}} Jim Green of NASA's Science Mission Directorate is optimistic that it could be observed by the [[w:James Webb Space Telescope#Infrared astronomy|James Webb Space Telescope]], the successor to the [[w:Hubble Space Telescope|Hubble Space Telescope]], that is expected to be launched in 2021.<ref name='popsci'/>

===Citizen science===
{{See also|w:Citizen science}}
The Zooniverse [[w:Backyard Worlds: Planet 9|Backyard Worlds]] project, started in February 2017, is using archival data from the WISE spacecraft to search for Planet Nine. The project will also search for substellar objects like [[w:brown dwarf|brown dwarf]]s in the neighborhood of the [[w:Solar System|Solar System]].<ref name="earthsky20170220">{{cite news |url=http://earthsky.org/space/help-astronomers-look-for-planet-9 |title=Help Astronomers Look for Planet 9 |work=[[EarthSky]] |first1=Deborah |last1=Byrd |first2=Eleanor |last2=Imster |date=20 February 2017 |accessdate=9 April 2017 |archive-url=https://web.archive.org/web/20170410062214/http://earthsky.org/space/help-astronomers-look-for-planet-9 |archive-date=10 April 2017 |df=dmy-all }}</ref><ref name="csmonitor20170217">{{cite news |url=http://www.csmonitor.com/Science/Spacebound/2017/0217/Hunt-for-Planet-9-how-you-can-help-NASA-search-for-brown-dwarfs-and-low-mass-stars |title=Hunt for Planet 9: How You Can Help NASA Search for Brown Dwarfs and Low-Mass Stars |work=[[The Christian Science Monitor]] |first=Story |last=Hinckley |date=17 February 2017 |accessdate=9 April 2017 |archive-url=https://web.archive.org/web/20170408225037/http://www.csmonitor.com/Science/Spacebound/2017/0217/Hunt-for-Planet-9-how-you-can-help-NASA-search-for-brown-dwarfs-and-low-mass-stars |archive-date=8 April 2017 |df=dmy-all }}</ref> 32,000 animations of four images each, which constitute 3 per cent of the WISE data, have been uploaded to the Backyard Worlds website. By looking for moving objects in the animations, citizen scientists might find Planet Nine.<ref name=findp9>{{cite web |last=Strom |first=Marcus |url=https://www.smh.com.au/technology/you-can-help-find-planet-nine-from-outer-space-through-citizen-science-20170216-gue44k.html|title=You Can Help Find Planet Nine from Outer Space Through Citizen Science|website=[[The Sydney Morning Herald]]|date=16 Feb 2017|accessdate=12 Nov 2018|archive-url=https://web.archive.org/web/20180618025523/https://www.smh.com.au/technology/you-can-help-find-planet-nine-from-outer-space-through-citizen-science-20170216-gue44k.html|archive-date=18 June 2018|df=dmy-all}}</ref>

In April 2017,<ref name="earthsky20170327">{{cite news |url=http://earthsky.org/space/anu-citizen-science-search-for-planet-9 |title=Another Planet 9 Search! You Can Help |work=[[EarthSky]] |first=Deborah |last=Byrd |date=27 March 2017 |accessdate=8 April 2017 |archive-url=https://web.archive.org/web/20170409131625/http://earthsky.org/space/anu-citizen-science-search-for-planet-9 |archive-date=9 April 2017 |df=dmy-all }}</ref> using data from the [[w:SkyMapper|SkyMapper]] telescope at [[w:Siding Spring Observatory|Siding Spring Observatory]], [[w:citizen scientist|citizen scientist]]s on the [[w:Zooniverse|Zooniverse]] platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability.<ref name="space20170403">{{cite news |url=http://www.space.com/36322-planet-nine-candidates-citizen-science.html |title=Where's Planet Nine? Citizen Scientists Spot 4 Possible Candidates |work=[[Space.com]] |first=Mike |last=Wall |date=3 April 2017 |accessdate=8 April 2017 |archive-url=https://web.archive.org/web/20170409112829/http://www.space.com/36322-planet-nine-candidates-citizen-science.html |archive-date=9 April 2017 |df=dmy-all }}</ref> The project, which started on 28 March 2017, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.<ref name="space20170403" />

==Attempts to predict location==
===Cassini measurements of Saturn's orbit===
Precise observations of Saturn's orbit using data from [[w:Cassini–Huygens|''Cassini'']] suggest that Planet Nine could not be in certain sections of its proposed orbit because its gravity would cause a noticeable effect on Saturn's position. This data neither proves nor disproves that Planet Nine exists.<ref name="Fienga_2016" />

An initial analysis by Fienga, Laskar, Manche, and Gastineau using Cassini data to search for Saturn's orbital residuals, small differences with its predicted orbit due to the Sun and the known planets, was inconsistent with Planet Nine being located with a [[w:true anomaly|true anomaly]], the location along its orbit relative to perihelion, of −130° to −110° or −65° to 85°. The analysis, using Batygin and Brown's orbital parameters for Planet Nine, suggests that the lack of perturbations to Saturn's orbit is best explained if Planet Nine is located at a true anomaly of {{val|117.8|+11|-10|u=deg}}. At this location, Planet Nine would be approximately {{val|630|u=AU}} from the Sun,<ref name=Fienga_2016>{{cite journal |last1=Fienga |first1=A. |last2=Laskar |first2=J. |last3=Manche |first3=H. |last4=Gastineau |first4=M. |date=2016 |title=Constraints on the Location of a Possible 9th Planet Derived from the Cassini Data |journal=[[Astronomy and Astrophysics]] |volume=587 |issue=1 |pages=L8 |arxiv=1602.06116 |bibcode=2016A&A...587L...8F |doi=10.1051/0004-6361/201628227}}</ref> with [[w:right ascension|right ascension]] close to 2<sup>h</sup> and [[w:declination|declination]] close to −20°, in Cetus.<ref name=findingNine>{{cite journal |first1=Carlos |last1=de la Fuente Marcos |first2=Raúl |last2=de la Fuente Marcos |year=2016 |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=459 |issue=1 |pages=L66–L70 |title=Finding Planet Nine: a Monte Carlo Approach |arxiv=1603.06520 |bibcode=2016MNRAS.459L..66D |doi=10.1093/mnrasl/slw049}}</ref> In contrast, if the putative planet is near aphelion it would be located near right ascension 3.0<sup>h</sup> to 5.5<sup>h</sup> and declination −1° to 6°.<ref name=peripole>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |year=2016 |title=Finding Planet Nine: Apsidal Anti-Alignment Monte Carlo Results |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=462 |issue=2 |pages=1972–1977 |arxiv=1607.05633 |bibcode=2016MNRAS.462.1972D |doi=10.1093/mnras/stw1778}}</ref>

A later analysis of ''Cassini'' data by astrophysicists Matthew Holman and Matthew Payne tightened the constraints on possible locations of Planet Nine. Holman and Payne developed a more efficient model that allowed them to explore a broader range of parameters than the previous analysis. The parameters identified using this technique to analyze the Cassini data was then intersected with Batygin and Brown's dynamical constraints on Planet Nine's orbit. Holman and Payne concluded that Planet Nine is most likely to be located within 20° of RA = 40°, Dec = −15°, in an area of the sky near the constellation Cetus.<ref name="Shannon_NewSci" /><ref name="Holman_Payne_Cassini">{{cite journal |last1=Holman |first1=Matthew J. |last2=Payne |first2=Matthew J. |title=Observational Constraints on Planet Nine: Cassini Range Observations |journal=[[The Astronomical Journal]] |date=2016 |volume=152 |issue=4 |page=94 |doi=10.3847/0004-6256/152/4/94 |arxiv=1604.03180 |bibcode=2016AJ....152...94H}}</ref>

William Folkner, a planetary scientist at the [[w:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] (JPL), has stated that the ''Cassini'' spacecraft is not experiencing unexplained deviations in its orbit around Saturn. An undiscovered planet would affect the orbit of Saturn, not ''Cassini''. This could produce a signature in the measurements of ''Cassini'', but JPL has seen no unexplained signatures in ''Cassini'' data.<ref name="JPL">{{cite web |date=8 April 2016 |title=Saturn Spacecraft Not Affected by Hypothetical Planet 9 |url=http://www.jpl.nasa.gov/news/news.php?feature=6200 |website=[[NASA]]/[[Jet Propulsion Laboratory]] |access-date=20 April 2016 |archive-url=https://web.archive.org/web/20160416140310/http://www.jpl.nasa.gov/news/news.php?feature=6200 |archive-date=16 April 2016 |df=dmy-all }}</ref>

===Analysis of Pluto's orbit===
An analysis in 2016 of Pluto's orbit by Holman and Payne found perturbations much larger than predicted by Batygin and Brown's proposed orbit for Planet Nine. Holman and Payne suggested three possible explanations: systematic errors in the measurements of Pluto's orbit; an unmodeled mass in the Solar System, such as a small planet in the range of 60–{{val|100|u=AU}} (potentially explaining the [[w:Kuiper cliff|Kuiper cliff]]); or a planet more massive or closer to the Sun instead of the planet predicted by Batygin and Brown.<ref name="sciamGiantGhost" /><ref name=Holman-Payne>{{cite journal |last1=Holman |first1=Matthew J. |last2=Payne |first2=Matthew J. |date=9 September 2016 |title=Observational Constraints on Planet Nine: Astrometry of Pluto and Other Trans-Neptunian Objects |journal=[[The Astronomical Journal]] |volume=152 |issue=4 |page=80 |arxiv=1603.09008 |bibcode=2016AJ....152...80H |doi=10.3847/0004-6256/152/4/80}}</ref>

===Orbits of nearly parabolic comets===
An analysis of the orbits of comets with nearly [[w:parabolic orbit|parabolic orbit]]s identifies five new comets with [[w:hyperbolic orbit|hyperbolic orbit]]s that approach the nominal orbit of Planet Nine described in Batygin and Brown's initial article. If these orbits are hyperbolic due to close encounters with Planet Nine the analysis estimates that Planet Nine is currently near aphelion with a right ascension of 83–90° and a declination of 8–10°.<ref name="Medvedev_etal_2017">{{cite journal |last1=Medvedev |first1=Yu D. |last2=Vavilov |first2=D.E. |last3=Bondarenko |first3=Yu S. |last4=Bulekbaev |first4=D.A. |last5=Kunturova |first5=N.B. |title=Improvement of the Position of Planet X Based on the Motion of Nearly Parabolic Comets |journal=[[Astronomy Letters]] |date=2017 |volume=42 |issue=2 |pages=120–125 |doi=10.1134/S1063773717020037 |bibcode=2017AstL...43..120M}}</ref> Scott Sheppard, who is skeptical of this analysis, notes that many different forces influence the orbits of comets.<ref name=sciamGiantGhost/>

===Occultations by Jupiter Trojans===
Malena Rice and Gregory Laughlin have proposed that a network of telescopes be built to detect occultations by Jupiter Trojans. The timing of these occultations would provide precise astrometry of these objects enabling their orbits to be monitored for variations due to the tide from Planet Nine.<ref name="Rice_Laughlin_2019">{{cite journal |last1=Rice |first1=Malena |last2=Laughlin |first2=Gregory |title=The Case for a Large-Scale Occultation Network |journal=The Astronomical Journal |volume=158 |issue=1 |pages=19 |date=2019 |arxiv=1905.06354|doi=10.3847/1538-3881/ab21df |bibcode=2019AJ....158...19R }}</ref>

==Attempts to predict semimajor axis==

An analysis by Sarah Millholland and Gregory Laughlin identified a pattern of [[w:Commensurability (astronomy)|commensurabilities]] (ratios between orbital periods of pairs of objects consistent with both being in resonance with another object) of the eTNOs. They identify five objects that would be near resonances with Planet Nine if had a semi-major axis of 654 AU: Sedna (3:2), {{mpl|2004 VN|112}} (3:1), {{mp|2012 VP|113}} (4:1), {{mpl|2000 CR|105}} (5:1), and {{mpl|2001 FP|185}} (5:1). They identify this planet as Planet Nine but propose a different orbit with an eccentricity ''e'' ≈ 0.5, inclination ''i'' ≈ 30°, argument of perihelion ω ≈ 150°, and longitude of ascending node Ω ≈ 50° (the last differs from Brown and Batygin's value of 90°).<ref name="Millholland_Laughlin_2017">{{cite journal |last1=Millholland |first1=Sarah |last2=Laughlin |first2=Gregory |title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-Motion Resonances |journal=[[The Astronomical Journal]] |date=2017 |volume=153 |issue=3 |page=91 |doi=10.3847/1538-3881/153/3/91 |arxiv=1612.07774 |bibcode=2017AJ....153...91M}}</ref>{{efn-ua|A 3-D version of the image of the orbit and those of several eTNOs shown in figure 14 of "Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-motion Resonances" is available.<ref name="Millholland_3D_orbit">{{cite journal |last1=Millholland |first1=Sarah |last2=Laughlin |first2=Gregory |title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-Motion Resonances |journal=[[The Astronomical Journal]] |date=2017 |volume=153 |issue=3 |page=91 |doi=10.3847/1538-3881/153/3/91 |arxiv=1612.07774 |bibcode=2017AJ....153...91M}} supplemented by {{cite web |last1=Millholland |first1=Sarah |title=Planet Nine's Orbit in Space |website=[[GitHub]] |url=https://smillholland.github.io/P9_Orbit/ |accessdate=8 August 2017 |archive-url=https://web.archive.org/web/20170221005255/https://smillholland.github.io/P9_Orbit/ |archive-date=21 February 2017 |df=dmy-all }}</ref>}}

Carlos and Raul de la Fuente Marcos also note commensurabilities among the known eTNOs similar to that of the Kuiper belt, where accidental commensurabilities occur due to objects in resonances with Neptune. They find that some of these objects would be in 5:3 and 3:1 resonances with a planet that had a semi-major axis of ≈700 AU.<ref name=moreplanets>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |year=2016 |title=Commensurabilities between ETNOs: a Monte Carlo survey |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=460 |issue=1 |pages=L64–L68 |arxiv=1604.05881 |bibcode=2016MNRAS.460L..64D |doi=10.1093/mnrasl/slw077}}</ref>

Three objects with smaller semi-major axes near 172 AU ({{mpl|2013 UH|15}}, {{mpl|2016 QV|89}} and {{mpl|2016 QU|89}}) have also been proposed to be in resonance with Planet Nine. These objects would be in resonance and anti-aligned with Planet Nine if it had a semi-major axis of 315 AU, below the range proposed by Batygin and Brown. Alternatively, they could be in resonance with Planet Nine, but have orbital orientations that circulate instead of being confined by Planet Nine if it had a semi-major axis of 505 AU.<ref name="Kaine_etal_2018">{{cite journal |last1=Kaine |first1=T.  |display-authors=etal|title=Dynamical Analysis of Three Distant Trans-Neptunian Objects with Similar Orbits |journal=The Astronomical Journal |date=2018 |volume=156 |issue=6 |page=273 |doi=10.3847/1538-3881/aaeb2a |arxiv=1810.10084 |bibcode=2018AJ....156..273K  }}</ref>

A later analysis by Elizabeth Bailey, Michael Brown and Konstantin Batygin found that if Planet Nine is in an eccentric and inclined orbit the capture of many of the eTNOs in higher order resonances and their chaotic transfer between resonances prevent the identification of Planet Nine's semi-major axis using current observations. They also determined that the odds of the first six objects observed being in N/1 or N/2 period ratios with Planet Nine are less than 5% if it has an eccentric orbit.<ref name="Bailey_etal_2018">{{cite journal |last1=Bailey |first1=Elizabeth |last2=Brown |first2=Michael E. |last3=Batygin |first3=Konstantin |title=Feasibility of a Resonance-Based Planet Nine Search |journal=[[The Astronomical Journal]] |date=2018 |volume=156 |issue=2 |page=74 |arxiv=1809.02594 |doi=10.3847/1538-3881/aaccf4|bibcode=2018AJ....156...74B }}</ref>

==Naming==
Planet Nine does not have an official name and will not receive one unless its existence is confirmed via imaging. Only two planets, Uranus and Neptune, have been discovered in recorded history. However, many [[w:minor planets|minor planets]], including [[w:dwarf planets|dwarf planets]], [[w:asteroids|asteroids]], and comets have been discovered and named. Consequently, there is [[w:Astronomical naming conventions|a well-established process]] for naming newly discovered solar system objects. If Planet Nine is observed, the [[w:International Astronomical Union|International Astronomical Union]] will certify a name, with priority usually given to a name proposed by its discoverers.<ref name="IAU">{{cite web |title=Naming of Astronomical Objects |url=https://www.iau.org/public/themes/naming/ |website=[[International Astronomical Union]] |accessdate=25 February 2016 |archive-url=https://web.archive.org/web/20160617214812/http://www.iau.org/public/themes/naming/ |archive-date=17 June 2016 |df=dmy-all }}</ref> It is likely to be a name chosen from [[w:Roman mythology|Roman]] or [[w:Greek mythology|Greek mythology]].<ref name=planetnames>{{cite web |last=Totten |first=Sanden |date=22 January 2016 |title=Planet 9: What Should Its Name Be If It's Found? |url=http://www.scpr.org/news/2016/01/22/56998/planet-9-what-should-its-name-be-if-it-s-found |website=[[89.3 KPCC]] |accessdate=7 February 2016 |quote={{-'}}We like to be consistent,' said Rosaly Lopes, a senior research scientist at NASA's Jet Propulsion Laboratory and a member of the IAU's Working Group for Planetary System Nomenclature. ... For a planet in our solar system, being consistent means sticking to the theme of giving them names from Greek and Roman mythology. |archive-url=https://web.archive.org/web/20160207085353/http://www.scpr.org/news/2016/01/22/56998/planet-9-what-should-its-name-be-if-it-s-found/ |archive-date=7 February 2016 |df=dmy-all }}</ref>

In their original article, Batygin and Brown simply referred to the object as "perturber",<ref name=TAJ201601 /> and only in later press releases did they use "Planet Nine".<ref name="Fesemaier_2016">{{cite web |last1=Fesenmaier |first1=Kimm |title=Caltech Researchers Find Evidence of a Real Ninth Planet | date=20 Jan 2016 |url=http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |website=[[California Institute of Technology|Caltech]] |accessdate=15 January 2019}}</ref> They have also used the names "[[w:Jehoshaphat|Jehoshaphat]]" and "George" for Planet Nine. Brown has stated: "We actually call it [[wikt:phattie|Phattie]]{{efn-ua|Most news outlets reported the name as ''[[wikt:phattie|Phattie]]'' (a slang term for "cool" or "awesome"; also, a marijuana cigarette)<ref name=sciencemag201601/> but ''The New Yorker'' quote cited above uses "fatty" in what appears to be a nearly unique variation. The apparently correct spelling has been substituted.}} when we're just talking to each other."<ref name=fatty />  In a 2019 interview with [[w:Derek Muller|Derek Muller]] for the [[w:YouTube channel|YouTube channel]] ''Veritasium'', Batygin also suggested, based on an online petition, to name the planet after singer [[w:David Bowie|David Bowie]], and to name any potential moons of the planet after characters from Bowie's song catalogue, such as [[w:Ziggy Stardust (character)|Ziggy Stardust]] or [[w:Starman (song)|Starman]].<ref>{{cite web |title=Does Planet 9 Exist? |url=https://youtube.com/pe83T9hISoY?t=915 |website=YouTube.com |accessdate=13 September 2019 |date=13 September 2019}}</ref>

In 2018, planetary scientist [[w:Alan Stern|Alan Stern]] objected to the name ''Planet Nine'', saying, "It is an effort to erase Clyde Tombaugh's legacy and it's frankly insulting", suggesting the name Planet X until its discovery.<ref name="Xv9">
{{cite web
 | url=http://www.businessinsider.com/planet-nine-x-name-argument-2018-6
 | title       = Is It Planet 9 or Planet X? Scientists Spar over What to Call the Solar System's Hypothetical Missing World
 | website   = [[Business Insider]]
 | first       = Dave
 | last        = Mosher
 | date        = June 7, 2018
 | access-date = 2018-06-09
 | archive-url = https://web.archive.org/web/20180608090727/http://www.businessinsider.com/planet-nine-x-name-argument-2018-6
 | archive-date= 8 June 2018
  | df          = dmy-all
}}</ref> He signed a statement with 34 other scientists saying, "We further believe the use of this term [Planet Nine] should be discontinued in favor of culturally and [[w:Taxonomy (general)|taxonomically]] neutral terms for such planets, such as Planet X, Planet Next, or Giant Planet Five."<ref>
{{cite journal
 | journal     = Planetary Exploration Newsletter
 | url         = http://planetarynews.org/archive18/pen_v12_n31_180729.txt
 | title       = On The Insensitive Use of the Term 'Planet 9' for Objects Beyond Pluto
 | author      = Paul Abell|display-authors=etal| volume      = 12
 | number      = 31
 | date        = 29 July 2018
 | accessdate = 2019-01-15
}}</ref> According to Brown, {{"'}}[[w:Planet X|Planet X]]' is not a generic reference to some unknown planet, but a specific prediction of Lowell's which led to the (accidental) discovery of Pluto. Our prediction is not related to this prediction."<ref name="Xv9"/>
==Notes==
{{reflist|group=upper-alpha}}

==References==
{{reflist|30em|refs=
<ref name="bnb20160617">{{cite journal |last1=Brown |first1=Michael E. |last2=Batygin |first2=Konstantin |title=Observational Constraints on the Orbit and Location of Planet Nine in the Outer Solar System |journal=[[The Astrophysical Journal Letters]] |date=2016 |volume=824 |issue=2 |page=L23 |doi=10.3847/2041-8205/824/2/L23 |arxiv=1603.05712 |bibcode=2016ApJ...824L..23B }}</ref><ref name="lurking">{{cite news
 |last         = Achenbach
 |first        = Joel
 |author-link  = Joel Achenbach
 |last2        = Feltman
 |first2       = Rachel
 |date         = 20 January 2016
 |title        = New Evidence Suggests a Ninth Planet Lurking at the Edge of the Solar System
 |url          = https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/20/new-evidence-suggests-a-ninth-planet-lurking-at-the-edge-of-the-solar-system/
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 |archive-date = 20 January 2016
  |df           = dmy-all
}}</ref>
<ref name="Gomes_Deienno_Morbidelli_2016">{{cite journal |last1=Gomes |first1=Rodney |last2=Deienno |first2=Rogerio |last3=Morbidelli |first3=Alessandro |title=The Inclination of the Planetary System Relative to the Solar Equator May Be Explained by the Presence of Planet 9 |date=2016 |journal=[[The Astronomical Journal]] |volume=153 |issue=1 |page=27 |arxiv=1607.05111 |doi=10.3847/1538-3881/153/1/27 |bibcode=2017AJ....153...27G}}</ref>
<ref name=CaltechJan2016>{{cite web |last=Fesenmaier |first=Kimm |date=20 January 2016 |title=Caltech Researchers Find Evidence of a Real Ninth Planet |url=https://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |website=[[California Institute of Technology|Caltech]] |accessdate=20 January 2016 |archive-url=https://web.archive.org/web/20160120165807/https://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |archive-date=20 January 2016 |df=dmy-all }}</ref>
<ref name=TAJ201601>{{cite journal |last1=Batygin |first1=Konstantin |author-link1=Konstantin Batygin |last2=Brown |first2=Michael E. |author-link2=Michael E. Brown |date=2016 |title=Evidence for a Distant Giant Planet in the Solar System |journal=[[The Astronomical Journal]] |volume=151 |issue=2 |page=22 |arxiv=1601.05438 |bibcode=2016AJ....151...22B |doi=10.3847/0004-6256/151/2/22 }}</ref>
<ref name="Beust_2016">{{cite journal |last1=Beust |first1=H. |title=Orbital Clustering of Distant Kuiper Belt Objects by Hypothetical Planet 9. Secular or Resonant? |journal=[[Astronomy & Astrophysics]] |date=2016 |volume=590 |page=L2 |doi=10.1051/0004-6361/201628638 |arxiv=1605.02473 |bibcode=2016A&A...590L...2B |df=dmy-all }}</ref>

<ref name=P9H2019>
{{cite journal
| last1 = Batygin
| first1 = Konstantin
| last2 = Adams
| first2 = Fred C.
| last3 = Brown 
| first3 = Michael E.
| last4 =  Becker
| first4= Juliette C.
| title      = The Planet Nine Hypothesis
| date       = 2019
| url        =
| journal    = [[Physics Reports]]
| volume     =805
| issue      =
| pages      =1–53
| doi        = 10.1016/j.physrep.2019.01.009
| arxiv = 1902.10103
| bibcode = 2019arXiv190210103B
}}</ref>

<ref name=NatGeo201601>{{cite magazine |first=Nadia |last=Drake |date=20 January 2016 |title=Scientists Find Evidence for Ninth Planet in Solar System |url=http://news.nationalgeographic.com/2016/01/150119-new-ninth-planet-solar-system-space/ |magazine=[[National Geographic (magazine)|National Geographic]] |accessdate=15 July 2016 |archive-url=https://web.archive.org/web/20160629022424/http://news.nationalgeographic.com/2016/01/150119-new-ninth-planet-solar-system-space/ |archive-date=29 June 2016 |df=dmy-all }}</ref>
}}